text: /*-------------------------------------------------------------------------
   *
   * predicate.c
   *	  POSTGRES predicate locking
   *	  to support full serializable transaction isolation
   *
   *
   * The approach taken is to implement Serializable Snapshot Isolation (SSI)
   * as initially described in this paper:
   *
   *	Michael J. Cahill, Uwe Röhm, and Alan D. Fekete. 2008.
   *	Serializable isolation for snapshot databases.
   *	In SIGMOD '08: Proceedings of the 2008 ACM SIGMOD
   *	international conference on Management of data,
   *	pages 729-738, New York, NY, USA. ACM.
   *	http://doi.acm.org/10.1145/1376616.1376690
   *
   * and further elaborated in Cahill's doctoral thesis:
   *
   *	Michael James Cahill. 2009.
   *	Serializable Isolation for Snapshot Databases.
   *	Sydney Digital Theses.
   *	University of Sydney, School of Information Technologies.
   *	http://hdl.handle.net/2123/5353
   *
   *
   * Predicate locks for Serializable Snapshot Isolation (SSI) are SIREAD
   * locks, which are so different from normal locks that a distinct set of
   * structures is required to handle them.  They are needed to detect
   * rw-conflicts when the read happens before the write.  (When the write
   * occurs first, the reading transaction can check for a conflict by
   * examining the MVCC data.)
   *
   * (1)	Besides tuples actually read, they must cover ranges of tuples
   *		which would have been read based on the predicate.  This will
   *		require modelling the predicates through locks against database
   *		objects such as pages, index ranges, or entire tables.
   *
   * (2)	They must be kept in RAM for quick access.  Because of this, it
   *		isn't possible to always maintain tuple-level granularity -- when
   *		the space allocated to store these approaches exhaustion, a
   *		request for a lock may need to scan for situations where a single
   *		transaction holds many fine-grained locks which can be coalesced
   *		into a single coarser-grained lock.
   *
   * (3)	They never block anything; they are more like flags than locks
   *		in that regard; although they refer to database objects and are
   *		used to identify rw-conflicts with normal write locks.
   *
   * (4)	While they are associated with a transaction, they must survive
   *		a successful COMMIT of that transaction, and remain until all
   *		overlapping transactions complete.  This even means that they
   *		must survive termination of the transaction's process.  If a
   *		top level transaction is rolled back, however, it is immediately
   *		flagged so that it can be ignored, and its SIREAD locks can be
   *		released any time after that.
   *
   * (5)	The only transactions which create SIREAD locks or check for
   *		conflicts with them are serializable transactions.
   *
   * (6)	When a write lock for a top level transaction is found to cover
   *		an existing SIREAD lock for the same transaction, the SIREAD lock
   *		can be deleted.
   *
   * (7)	A write from a serializable transaction must ensure that an xact
   *		record exists for the transaction, with the same lifespan (until
   *		all concurrent transaction complete or the transaction is rolled
   *		back) so that rw-dependencies to that transaction can be
   *		detected.
   *
   * We use an optimization for read-only transactions. Under certain
   * circumstances, a read-only transaction's snapshot can be shown to
   * never have conflicts with other transactions.  This is referred to
   * as a "safe" snapshot (and one known not to be is "unsafe").
   * However, it can't be determined whether a snapshot is safe until
   * all concurrent read/write transactions complete.
   *
   * Once a read-only transaction is known to have a safe snapshot, it
   * can release its predicate locks and exempt itself from further
   * predicate lock tracking. READ ONLY DEFERRABLE transactions run only
   * on safe snapshots, waiting as necessary for one to be available.
   *
   *
   * Lightweight locks to manage access to the predicate locking shared
   * memory objects must be taken in this order, and should be released in
   * reverse order:
   *
   *	SerializableFinishedListLock
   *		- Protects the list of transactions which have completed but which
   *			may yet matter because they overlap still-active transactions.
   *
   *	SerializablePredicateLockListLock
   *		- Protects the linked list of locks held by a transaction.  Note
   *			that the locks themselves are also covered by the partition
   *			locks of their respective lock targets; this lock only affects
   *			the linked list connecting the locks related to a transaction.
   *		- All transactions share this single lock (with no partitioning).
   *		- There is never a need for a process other than the one running
   *			an active transaction to walk the list of locks held by that
   *			transaction.
   *		- It is relatively infrequent that another process needs to
   *			modify the list for a transaction, but it does happen for such
   *			things as index page splits for pages with predicate locks and
   *			freeing of predicate locked pages by a vacuum process.  When
   *			removing a lock in such cases, the lock itself contains the
   *			pointers needed to remove it from the list.  When adding a
   *			lock in such cases, the lock can be added using the anchor in
   *			the transaction structure.  Neither requires walking the list.
   *		- Cleaning up the list for a terminated transaction is sometimes
   *			not done on a retail basis, in which case no lock is required.
   *		- Due to the above, a process accessing its active transaction's
   *			list always uses a shared lock, regardless of whether it is
   *			walking or maintaining the list.  This improves concurrency
   *			for the common access patterns.
   *		- A process which needs to alter the list of a transaction other
   *			than its own active transaction must acquire an exclusive
   *			lock.
   *
   *	PredicateLockHashPartitionLock(hashcode)
   *		- The same lock protects a target, all locks on that target, and
   *			the linked list of locks on the target.
   *		- When more than one is needed, acquire in ascending address order.
   *		- When all are needed (rare), acquire in ascending index order with
   *			PredicateLockHashPartitionLockByIndex(index).
   *
   *	SerializableXactHashLock
   *		- Protects both PredXact and SerializableXidHash.
   *
   *
   * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/storage/lmgr/predicate.c
   *
   *-------------------------------------------------------------------------
   */
  /*
   * INTERFACE ROUTINES
   *
   * housekeeping for setting up shared memory predicate lock structures
   *		InitPredicateLocks(void)
   *		PredicateLockShmemSize(void)
   *
   * predicate lock reporting
   *		GetPredicateLockStatusData(void)
   *		PageIsPredicateLocked(Relation relation, BlockNumber blkno)
   *
   * predicate lock maintenance
   *		GetSerializableTransactionSnapshot(Snapshot snapshot)
   *		SetSerializableTransactionSnapshot(Snapshot snapshot,
   *										   VirtualTransactionId *sourcevxid)
   *		RegisterPredicateLockingXid(void)
   *		PredicateLockRelation(Relation relation, Snapshot snapshot)
   *		PredicateLockPage(Relation relation, BlockNumber blkno,
   *						Snapshot snapshot)
   *		PredicateLockTuple(Relation relation, HeapTuple tuple,
   *						Snapshot snapshot)
   *		PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
   *							   BlockNumber newblkno)
   *		PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
   *								 BlockNumber newblkno)
   *		TransferPredicateLocksToHeapRelation(Relation relation)
   *		ReleasePredicateLocks(bool isCommit)
   *
   * conflict detection (may also trigger rollback)
   *		CheckForSerializableConflictOut(bool visible, Relation relation,
   *										HeapTupleData *tup, Buffer buffer,
   *										Snapshot snapshot)
   *		CheckForSerializableConflictIn(Relation relation, HeapTupleData *tup,
   *									   Buffer buffer)
   *		CheckTableForSerializableConflictIn(Relation relation)
   *
   * final rollback checking
   *		PreCommit_CheckForSerializationFailure(void)
   *
   * two-phase commit support
   *		AtPrepare_PredicateLocks(void);
   *		PostPrepare_PredicateLocks(TransactionId xid);
   *		PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
   *		predicatelock_twophase_recover(TransactionId xid, uint16 info,
   *									   void *recdata, uint32 len);
   */
  
  #include "postgres.h"
  
  #include "access/htup_details.h"
  #include "access/slru.h"
  #include "access/subtrans.h"
  #include "access/transam.h"
  #include "access/twophase.h"
  #include "access/twophase_rmgr.h"
  #include "access/xact.h"
  #include "access/xlog.h"
  #include "miscadmin.h"
  #include "pgstat.h"
  #include "storage/bufmgr.h"
  #include "storage/predicate.h"
  #include "storage/predicate_internals.h"
  #include "storage/proc.h"
  #include "storage/procarray.h"
  #include "utils/rel.h"
  #include "utils/snapmgr.h"
  #include "utils/tqual.h"
  
  /* Uncomment the next line to test the graceful degradation code. */
  /* #define TEST_OLDSERXID */
  
  /*
   * Test the most selective fields first, for performance.
   *
   * a is covered by b if all of the following hold:
   *	1) a.database = b.database
   *	2) a.relation = b.relation
   *	3) b.offset is invalid (b is page-granularity or higher)
   *	4) either of the following:
   *		4a) a.offset is valid (a is tuple-granularity) and a.page = b.page
   *	 or 4b) a.offset is invalid and b.page is invalid (a is
   *			page-granularity and b is relation-granularity
   */
  #define TargetTagIsCoveredBy(covered_target, covering_target)			\
  	((GET_PREDICATELOCKTARGETTAG_RELATION(covered_target) == /* (2) */	\
  	  GET_PREDICATELOCKTARGETTAG_RELATION(covering_target))				\
  	 && (GET_PREDICATELOCKTARGETTAG_OFFSET(covering_target) ==			\
  		 InvalidOffsetNumber)								 /* (3) */	\
  	 && (((GET_PREDICATELOCKTARGETTAG_OFFSET(covered_target) !=			\
  		   InvalidOffsetNumber)								 /* (4a) */ \
  		  && (GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) ==		\
  			  GET_PREDICATELOCKTARGETTAG_PAGE(covered_target)))			\
  		 || ((GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) ==		\
  			  InvalidBlockNumber)							 /* (4b) */ \
  			 && (GET_PREDICATELOCKTARGETTAG_PAGE(covered_target)		\
  				 != InvalidBlockNumber)))								\
  	 && (GET_PREDICATELOCKTARGETTAG_DB(covered_target) ==	 /* (1) */	\
  		 GET_PREDICATELOCKTARGETTAG_DB(covering_target)))
  
  /*
   * The predicate locking target and lock shared hash tables are partitioned to
   * reduce contention.  To determine which partition a given target belongs to,
   * compute the tag's hash code with PredicateLockTargetTagHashCode(), then
   * apply one of these macros.
   * NB: NUM_PREDICATELOCK_PARTITIONS must be a power of 2!
   */
  #define PredicateLockHashPartition(hashcode) \
  	((hashcode) % NUM_PREDICATELOCK_PARTITIONS)
  #define PredicateLockHashPartitionLock(hashcode) \
  	(&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + \
  		PredicateLockHashPartition(hashcode)].lock)
  #define PredicateLockHashPartitionLockByIndex(i) \
  	(&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + (i)].lock)
  
  #define NPREDICATELOCKTARGETENTS() \
  	mul_size(max_predicate_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))
  
  #define SxactIsOnFinishedList(sxact) (!SHMQueueIsDetached(&((sxact)->finishedLink)))
  
  /*
   * Note that a sxact is marked "prepared" once it has passed
   * PreCommit_CheckForSerializationFailure, even if it isn't using
   * 2PC. This is the point at which it can no longer be aborted.
   *
   * The PREPARED flag remains set after commit, so SxactIsCommitted
   * implies SxactIsPrepared.
   */
  #define SxactIsCommitted(sxact) (((sxact)->flags & SXACT_FLAG_COMMITTED) != 0)
  #define SxactIsPrepared(sxact) (((sxact)->flags & SXACT_FLAG_PREPARED) != 0)
  #define SxactIsRolledBack(sxact) (((sxact)->flags & SXACT_FLAG_ROLLED_BACK) != 0)
  #define SxactIsDoomed(sxact) (((sxact)->flags & SXACT_FLAG_DOOMED) != 0)
  #define SxactIsReadOnly(sxact) (((sxact)->flags & SXACT_FLAG_READ_ONLY) != 0)
  #define SxactHasSummaryConflictIn(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_IN) != 0)
  #define SxactHasSummaryConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_OUT) != 0)
  /*
   * The following macro actually means that the specified transaction has a
   * conflict out *to a transaction which committed ahead of it*.  It's hard
   * to get that into a name of a reasonable length.
   */
  #define SxactHasConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_CONFLICT_OUT) != 0)
  #define SxactIsDeferrableWaiting(sxact) (((sxact)->flags & SXACT_FLAG_DEFERRABLE_WAITING) != 0)
  #define SxactIsROSafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_SAFE) != 0)
  #define SxactIsROUnsafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_UNSAFE) != 0)
  
  /*
   * Compute the hash code associated with a PREDICATELOCKTARGETTAG.
   *
   * To avoid unnecessary recomputations of the hash code, we try to do this
   * just once per function, and then pass it around as needed.  Aside from
   * passing the hashcode to hash_search_with_hash_value(), we can extract
   * the lock partition number from the hashcode.
   */
  #define PredicateLockTargetTagHashCode(predicatelocktargettag) \
  	get_hash_value(PredicateLockTargetHash, predicatelocktargettag)
  
  /*
   * Given a predicate lock tag, and the hash for its target,
   * compute the lock hash.
   *
   * To make the hash code also depend on the transaction, we xor the sxid
   * struct's address into the hash code, left-shifted so that the
   * partition-number bits don't change.  Since this is only a hash, we
   * don't care if we lose high-order bits of the address; use an
   * intermediate variable to suppress cast-pointer-to-int warnings.
   */
  #define PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash) \
  	((targethash) ^ ((uint32) PointerGetDatum((predicatelocktag)->myXact)) \
  	 << LOG2_NUM_PREDICATELOCK_PARTITIONS)
  
  
  /*
   * The SLRU buffer area through which we access the old xids.
   */
  static SlruCtlData OldSerXidSlruCtlData;
  
  #define OldSerXidSlruCtl			(&OldSerXidSlruCtlData)
  
  #define OLDSERXID_PAGESIZE			BLCKSZ
  #define OLDSERXID_ENTRYSIZE			sizeof(SerCommitSeqNo)
  #define OLDSERXID_ENTRIESPERPAGE	(OLDSERXID_PAGESIZE / OLDSERXID_ENTRYSIZE)
  
  /*
   * Set maximum pages based on the lesser of the number needed to track all
   * transactions and the maximum that SLRU supports.
   */
  #define OLDSERXID_MAX_PAGE			Min(SLRU_PAGES_PER_SEGMENT * 0x10000 - 1, \
  										(MaxTransactionId) / OLDSERXID_ENTRIESPERPAGE)
  
  #define OldSerXidNextPage(page) (((page) >= OLDSERXID_MAX_PAGE) ? 0 : (page) + 1)
  
  #define OldSerXidValue(slotno, xid) (*((SerCommitSeqNo *) \
  	(OldSerXidSlruCtl->shared->page_buffer[slotno] + \
  	((((uint32) (xid)) % OLDSERXID_ENTRIESPERPAGE) * OLDSERXID_ENTRYSIZE))))
  
  #define OldSerXidPage(xid)	((((uint32) (xid)) / OLDSERXID_ENTRIESPERPAGE) % (OLDSERXID_MAX_PAGE + 1))
  #define OldSerXidSegment(page)	((page) / SLRU_PAGES_PER_SEGMENT)
  
  typedef struct OldSerXidControlData
  {
  	int			headPage;		/* newest initialized page */
  	TransactionId headXid;		/* newest valid Xid in the SLRU */
  	TransactionId tailXid;		/* oldest xmin we might be interested in */
  	bool		warningIssued;	/* have we issued SLRU wrap-around warning? */
  }			OldSerXidControlData;
  
  typedef struct OldSerXidControlData *OldSerXidControl;
  
  static OldSerXidControl oldSerXidControl;
  
  /*
   * When the oldest committed transaction on the "finished" list is moved to
   * SLRU, its predicate locks will be moved to this "dummy" transaction,
   * collapsing duplicate targets.  When a duplicate is found, the later
   * commitSeqNo is used.
   */
  static SERIALIZABLEXACT *OldCommittedSxact;
  
  
  /*
   * These configuration variables are used to set the predicate lock table size
   * and to control promotion of predicate locks to coarser granularity in an
   * attempt to degrade performance (mostly as false positive serialization
   * failure) gracefully in the face of memory pressurel
   */
  int			max_predicate_locks_per_xact;	/* set by guc.c */
  int			max_predicate_locks_per_relation;	/* set by guc.c */
  int			max_predicate_locks_per_page;	/* set by guc.c */
  
  /*
   * This provides a list of objects in order to track transactions
   * participating in predicate locking.  Entries in the list are fixed size,
   * and reside in shared memory.  The memory address of an entry must remain
   * fixed during its lifetime.  The list will be protected from concurrent
   * update externally; no provision is made in this code to manage that.  The
   * number of entries in the list, and the size allowed for each entry is
   * fixed upon creation.
   */
  static PredXactList PredXact;
  
  /*
   * This provides a pool of RWConflict data elements to use in conflict lists
   * between transactions.
   */
  static RWConflictPoolHeader RWConflictPool;
  
  /*
   * The predicate locking hash tables are in shared memory.
   * Each backend keeps pointers to them.
   */
  static HTAB *SerializableXidHash;
  static HTAB *PredicateLockTargetHash;
  static HTAB *PredicateLockHash;
  static SHM_QUEUE *FinishedSerializableTransactions;
  
  /*
   * Tag for a dummy entry in PredicateLockTargetHash. By temporarily removing
   * this entry, you can ensure that there's enough scratch space available for
   * inserting one entry in the hash table. This is an otherwise-invalid tag.
   */
  static const PREDICATELOCKTARGETTAG ScratchTargetTag = {0, 0, 0, 0};
  static uint32 ScratchTargetTagHash;
  static LWLock *ScratchPartitionLock;
  
  /*
   * The local hash table used to determine when to combine multiple fine-
   * grained locks into a single courser-grained lock.
   */
  static HTAB *LocalPredicateLockHash = NULL;
  
  /*
   * Keep a pointer to the currently-running serializable transaction (if any)
   * for quick reference. Also, remember if we have written anything that could
   * cause a rw-conflict.
   */
  static SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;
  static bool MyXactDidWrite = false;
  
  /* local functions */
  
  static SERIALIZABLEXACT *CreatePredXact(void);
  static void ReleasePredXact(SERIALIZABLEXACT *sxact);
  static SERIALIZABLEXACT *FirstPredXact(void);
  static SERIALIZABLEXACT *NextPredXact(SERIALIZABLEXACT *sxact);
  
  static bool RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer);
  static void SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
  static void SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact, SERIALIZABLEXACT *activeXact);
  static void ReleaseRWConflict(RWConflict conflict);
  static void FlagSxactUnsafe(SERIALIZABLEXACT *sxact);
  
  static bool OldSerXidPagePrecedesLogically(int p, int q);
  static void OldSerXidInit(void);
  static void OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo);
  static SerCommitSeqNo OldSerXidGetMinConflictCommitSeqNo(TransactionId xid);
  static void OldSerXidSetActiveSerXmin(TransactionId xid);
  
  static uint32 predicatelock_hash(const void *key, Size keysize);
  static void SummarizeOldestCommittedSxact(void);
  static Snapshot GetSafeSnapshot(Snapshot snapshot);
  static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot,
  									  VirtualTransactionId *sourcevxid,
  									  int sourcepid);
  static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
  static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent);
  static bool CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag);
  static void RemoveScratchTarget(bool lockheld);
  static void RestoreScratchTarget(bool lockheld);
  static void RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target,
  						   uint32 targettaghash);
  static void DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag);
  static int	MaxPredicateChildLocks(const PREDICATELOCKTARGETTAG *tag);
  static bool CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag);
  static void DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag);
  static void CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
  					uint32 targettaghash,
  					SERIALIZABLEXACT *sxact);
  static void DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash);
  static bool TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
  								  PREDICATELOCKTARGETTAG newtargettag,
  								  bool removeOld);
  static void PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag);
  static void DropAllPredicateLocksFromTable(Relation relation,
  							   bool transfer);
  static void SetNewSxactGlobalXmin(void);
  static void ClearOldPredicateLocks(void);
  static void ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
  						   bool summarize);
  static bool XidIsConcurrent(TransactionId xid);
  static void CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag);
  static void FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
  static void OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
  										SERIALIZABLEXACT *writer);
  
  
  /*------------------------------------------------------------------------*/
  
  /*
   * Does this relation participate in predicate locking? Temporary and system
   * relations are exempt, as are materialized views.
   */
  static inline bool
  PredicateLockingNeededForRelation(Relation relation)
  {
  	return !(relation->rd_id < FirstBootstrapObjectId ||
  			 RelationUsesLocalBuffers(relation) ||
  			 relation->rd_rel->relkind == RELKIND_MATVIEW);
  }
  
  /*
   * When a public interface method is called for a read, this is the test to
   * see if we should do a quick return.
   *
   * Note: this function has side-effects! If this transaction has been flagged
   * as RO-safe since the last call, we release all predicate locks and reset
   * MySerializableXact. That makes subsequent calls to return quickly.
   *
   * This is marked as 'inline' to make to eliminate the function call overhead
   * in the common case that serialization is not needed.
   */
  static inline bool
  SerializationNeededForRead(Relation relation, Snapshot snapshot)
  {
  	/* Nothing to do if this is not a serializable transaction */
  	if (MySerializableXact == InvalidSerializableXact)
  		return false;
  
  	/*
  	 * Don't acquire locks or conflict when scanning with a special snapshot.
  	 * This excludes things like CLUSTER and REINDEX. They use the wholesale
  	 * functions TransferPredicateLocksToHeapRelation() and
  	 * CheckTableForSerializableConflictIn() to participate in serialization,
  	 * but the scans involved don't need serialization.
  	 */
  	if (!IsMVCCSnapshot(snapshot))
  		return false;
  
  	/*
  	 * Check if we have just become "RO-safe". If we have, immediately release
  	 * all locks as they're not needed anymore. This also resets
  	 * MySerializableXact, so that subsequent calls to this function can exit
  	 * quickly.
  	 *
  	 * A transaction is flagged as RO_SAFE if all concurrent R/W transactions
  	 * commit without having conflicts out to an earlier snapshot, thus
  	 * ensuring that no conflicts are possible for this transaction.
  	 */
  	if (SxactIsROSafe(MySerializableXact))
  	{
  		ReleasePredicateLocks(false);
  		return false;
  	}
  
  	/* Check if the relation doesn't participate in predicate locking */
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
  	return true;				/* no excuse to skip predicate locking */
  }
  
  /*
   * Like SerializationNeededForRead(), but called on writes.
   * The logic is the same, but there is no snapshot and we can't be RO-safe.
   */
  static inline bool
  SerializationNeededForWrite(Relation relation)
  {
  	/* Nothing to do if this is not a serializable transaction */
  	if (MySerializableXact == InvalidSerializableXact)
  		return false;
  
  	/* Check if the relation doesn't participate in predicate locking */
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
  	return true;				/* no excuse to skip predicate locking */
  }
  
  
  /*------------------------------------------------------------------------*/
  
  /*
   * These functions are a simple implementation of a list for this specific
   * type of struct.  If there is ever a generalized shared memory list, we
   * should probably switch to that.
   */
  static SERIALIZABLEXACT *
  CreatePredXact(void)
  {
  	PredXactListElement ptle;
  
  	ptle = (PredXactListElement)
  		SHMQueueNext(&PredXact->availableList,
  					 &PredXact->availableList,
  					 offsetof(PredXactListElementData, link));
  	if (!ptle)
  		return NULL;
  
  	SHMQueueDelete(&ptle->link);
  	SHMQueueInsertBefore(&PredXact->activeList, &ptle->link);
  	return &ptle->sxact;
  }
  
  static void
  ReleasePredXact(SERIALIZABLEXACT *sxact)
  {
  	PredXactListElement ptle;
  
  	Assert(ShmemAddrIsValid(sxact));
  
  	ptle = (PredXactListElement)
  		(((char *) sxact)
  		 - offsetof(PredXactListElementData, sxact)
  		 + offsetof(PredXactListElementData, link));
  	SHMQueueDelete(&ptle->link);
  	SHMQueueInsertBefore(&PredXact->availableList, &ptle->link);
  }
  
  static SERIALIZABLEXACT *
  FirstPredXact(void)
  {
  	PredXactListElement ptle;
  
  	ptle = (PredXactListElement)
  		SHMQueueNext(&PredXact->activeList,
  					 &PredXact->activeList,
  					 offsetof(PredXactListElementData, link));
  	if (!ptle)
  		return NULL;
  
  	return &ptle->sxact;
  }
  
  static SERIALIZABLEXACT *
  NextPredXact(SERIALIZABLEXACT *sxact)
  {
  	PredXactListElement ptle;
  
  	Assert(ShmemAddrIsValid(sxact));
  
  	ptle = (PredXactListElement)
  		(((char *) sxact)
  		 - offsetof(PredXactListElementData, sxact)
  		 + offsetof(PredXactListElementData, link));
  	ptle = (PredXactListElement)
  		SHMQueueNext(&PredXact->activeList,
  					 &ptle->link,
  					 offsetof(PredXactListElementData, link));
  	if (!ptle)
  		return NULL;
  
  	return &ptle->sxact;
  }
  
  /*------------------------------------------------------------------------*/
  
  /*
   * These functions manage primitive access to the RWConflict pool and lists.
   */
  static bool
  RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer)
  {
  	RWConflict	conflict;
  
  	Assert(reader != writer);
  
  	/* Check the ends of the purported conflict first. */
  	if (SxactIsDoomed(reader)
  		|| SxactIsDoomed(writer)
  		|| SHMQueueEmpty(&reader->outConflicts)
  		|| SHMQueueEmpty(&writer->inConflicts))
  		return false;
  
  	/* A conflict is possible; walk the list to find out. */
  	conflict = (RWConflict)
  		SHMQueueNext(&reader->outConflicts,
  					 &reader->outConflicts,
  					 offsetof(RWConflictData, outLink));
  	while (conflict)
  	{
  		if (conflict->sxactIn == writer)
  			return true;
  		conflict = (RWConflict)
  			SHMQueueNext(&reader->outConflicts,
  						 &conflict->outLink,
  						 offsetof(RWConflictData, outLink));
  	}
  
  	/* No conflict found. */
  	return false;
  }
  
  static void
  SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
  {
  	RWConflict	conflict;
  
  	Assert(reader != writer);
  	Assert(!RWConflictExists(reader, writer));
  
  	conflict = (RWConflict)
  		SHMQueueNext(&RWConflictPool->availableList,
  					 &RWConflictPool->availableList,
  					 offsetof(RWConflictData, outLink));
  	if (!conflict)
  		ereport(ERROR,
  				(errcode(ERRCODE_OUT_OF_MEMORY),
  				 errmsg("not enough elements in RWConflictPool to record a read/write conflict"),
  				 errhint("You might need to run fewer transactions at a time or increase max_connections.")));
  
  	SHMQueueDelete(&conflict->outLink);
  
  	conflict->sxactOut = reader;
  	conflict->sxactIn = writer;
  	SHMQueueInsertBefore(&reader->outConflicts, &conflict->outLink);
  	SHMQueueInsertBefore(&writer->inConflicts, &conflict->inLink);
  }
  
  static void
  SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact,
  						  SERIALIZABLEXACT *activeXact)
  {
  	RWConflict	conflict;
  
  	Assert(roXact != activeXact);
  	Assert(SxactIsReadOnly(roXact));
  	Assert(!SxactIsReadOnly(activeXact));
  
  	conflict = (RWConflict)
  		SHMQueueNext(&RWConflictPool->availableList,
  					 &RWConflictPool->availableList,
  					 offsetof(RWConflictData, outLink));
  	if (!conflict)
  		ereport(ERROR,
  				(errcode(ERRCODE_OUT_OF_MEMORY),
  				 errmsg("not enough elements in RWConflictPool to record a potential read/write conflict"),
  				 errhint("You might need to run fewer transactions at a time or increase max_connections.")));
  
  	SHMQueueDelete(&conflict->outLink);
  
  	conflict->sxactOut = activeXact;
  	conflict->sxactIn = roXact;
  	SHMQueueInsertBefore(&activeXact->possibleUnsafeConflicts,
  						 &conflict->outLink);
  	SHMQueueInsertBefore(&roXact->possibleUnsafeConflicts,
  						 &conflict->inLink);
  }
  
  static void
  ReleaseRWConflict(RWConflict conflict)
  {
  	SHMQueueDelete(&conflict->inLink);
  	SHMQueueDelete(&conflict->outLink);
  	SHMQueueInsertBefore(&RWConflictPool->availableList, &conflict->outLink);
  }
  
  static void
  FlagSxactUnsafe(SERIALIZABLEXACT *sxact)
  {
  	RWConflict	conflict,
  				nextConflict;
  
  	Assert(SxactIsReadOnly(sxact));
  	Assert(!SxactIsROSafe(sxact));
  
  	sxact->flags |= SXACT_FLAG_RO_UNSAFE;
  
  	/*
  	 * We know this isn't a safe snapshot, so we can stop looking for other
  	 * potential conflicts.
  	 */
  	conflict = (RWConflict)
  		SHMQueueNext(&sxact->possibleUnsafeConflicts,
  					 &sxact->possibleUnsafeConflicts,
  					 offsetof(RWConflictData, inLink));
  	while (conflict)
  	{
  		nextConflict = (RWConflict)
  			SHMQueueNext(&sxact->possibleUnsafeConflicts,
  						 &conflict->inLink,
  						 offsetof(RWConflictData, inLink));
  
  		Assert(!SxactIsReadOnly(conflict->sxactOut));
  		Assert(sxact == conflict->sxactIn);
  
  		ReleaseRWConflict(conflict);
  
  		conflict = nextConflict;
  	}
  }
  
  /*------------------------------------------------------------------------*/
  
  /*
   * We will work on the page range of 0..OLDSERXID_MAX_PAGE.
   * Compares using wraparound logic, as is required by slru.c.
   */
  static bool
  OldSerXidPagePrecedesLogically(int p, int q)
  {
  	int			diff;
  
  	/*
  	 * We have to compare modulo (OLDSERXID_MAX_PAGE+1)/2.  Both inputs should
  	 * be in the range 0..OLDSERXID_MAX_PAGE.
  	 */
  	Assert(p >= 0 && p <= OLDSERXID_MAX_PAGE);
  	Assert(q >= 0 && q <= OLDSERXID_MAX_PAGE);
  
  	diff = p - q;
  	if (diff >= ((OLDSERXID_MAX_PAGE + 1) / 2))
  		diff -= OLDSERXID_MAX_PAGE + 1;
  	else if (diff < -((int) (OLDSERXID_MAX_PAGE + 1) / 2))
  		diff += OLDSERXID_MAX_PAGE + 1;
  	return diff < 0;
  }
  
  /*
   * Initialize for the tracking of old serializable committed xids.
   */
  static void
  OldSerXidInit(void)
  {
  	bool		found;
  
  	/*
  	 * Set up SLRU management of the pg_serial data.
  	 */
  	OldSerXidSlruCtl->PagePrecedes = OldSerXidPagePrecedesLogically;
  	SimpleLruInit(OldSerXidSlruCtl, "oldserxid",
  				  NUM_OLDSERXID_BUFFERS, 0, OldSerXidLock, "pg_serial",
  				  LWTRANCHE_OLDSERXID_BUFFERS);
  	/* Override default assumption that writes should be fsync'd */
  	OldSerXidSlruCtl->do_fsync = false;
  
  	/*
  	 * Create or attach to the OldSerXidControl structure.
  	 */
  	oldSerXidControl = (OldSerXidControl)
  		ShmemInitStruct("OldSerXidControlData", sizeof(OldSerXidControlData), &found);
  
  	Assert(found == IsUnderPostmaster);
  	if (!found)
  	{
  		/*
  		 * Set control information to reflect empty SLRU.
  		 */
  		oldSerXidControl->headPage = -1;
  		oldSerXidControl->headXid = InvalidTransactionId;
  		oldSerXidControl->tailXid = InvalidTransactionId;
  		oldSerXidControl->warningIssued = false;
  	}
  }
  
  /*
   * Record a committed read write serializable xid and the minimum
   * commitSeqNo of any transactions to which this xid had a rw-conflict out.
   * An invalid seqNo means that there were no conflicts out from xid.
   */
  static void
  OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
  {
  	TransactionId tailXid;
  	int			targetPage;
  	int			slotno;
  	int			firstZeroPage;
  	bool		isNewPage;
  
  	Assert(TransactionIdIsValid(xid));
  
  	targetPage = OldSerXidPage(xid);
  
  	LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);
  
  	/*
  	 * If no serializable transactions are active, there shouldn't be anything
  	 * to push out to the SLRU.  Hitting this assert would mean there's
  	 * something wrong with the earlier cleanup logic.
  	 */
  	tailXid = oldSerXidControl->tailXid;
  	Assert(TransactionIdIsValid(tailXid));
  
  	/*
  	 * If the SLRU is currently unused, zero out the whole active region from
  	 * tailXid to headXid before taking it into use. Otherwise zero out only
  	 * any new pages that enter the tailXid-headXid range as we advance
  	 * headXid.
  	 */
  	if (oldSerXidControl->headPage < 0)
  	{
  		firstZeroPage = OldSerXidPage(tailXid);
  		isNewPage = true;
  	}
  	else
  	{
  		firstZeroPage = OldSerXidNextPage(oldSerXidControl->headPage);
  		isNewPage = OldSerXidPagePrecedesLogically(oldSerXidControl->headPage,
  												   targetPage);
  	}
  
  	if (!TransactionIdIsValid(oldSerXidControl->headXid)
  		|| TransactionIdFollows(xid, oldSerXidControl->headXid))
  		oldSerXidControl->headXid = xid;
  	if (isNewPage)
  		oldSerXidControl->headPage = targetPage;
  
  	/*
  	 * Give a warning if we're about to run out of SLRU pages.
  	 *
  	 * slru.c has a maximum of 64k segments, with 32 (SLRU_PAGES_PER_SEGMENT)
  	 * pages each. We need to store a 64-bit integer for each Xid, and with
  	 * default 8k block size, 65536*32 pages is only enough to cover 2^30
  	 * XIDs. If we're about to hit that limit and wrap around, warn the user.
  	 *
  	 * To avoid spamming the user, we only give one warning when we've used 1
  	 * billion XIDs, and stay silent until the situation is fixed and the
  	 * number of XIDs used falls below 800 million again.
  	 *
  	 * XXX: We have no safeguard to actually *prevent* the wrap-around,
  	 * though. All you get is a warning.
  	 */
  	if (oldSerXidControl->warningIssued)
  	{
  		TransactionId lowWatermark;
  
  		lowWatermark = tailXid + 800000000;
  		if (lowWatermark < FirstNormalTransactionId)
  			lowWatermark = FirstNormalTransactionId;
  		if (TransactionIdPrecedes(xid, lowWatermark))
  			oldSerXidControl->warningIssued = false;
  	}
  	else
  	{
  		TransactionId highWatermark;
  
  		highWatermark = tailXid + 1000000000;
  		if (highWatermark < FirstNormalTransactionId)
  			highWatermark = FirstNormalTransactionId;
  		if (TransactionIdFollows(xid, highWatermark))
  		{
  			oldSerXidControl->warningIssued = true;
  			ereport(WARNING,
  					(errmsg("memory for serializable conflict tracking is nearly exhausted"),
  					 errhint("There might be an idle transaction or a forgotten prepared transaction causing this.")));
  		}
  	}
  
  	if (isNewPage)
  	{
  		/* Initialize intervening pages. */
  		while (firstZeroPage != targetPage)
  		{
  			(void) SimpleLruZeroPage(OldSerXidSlruCtl, firstZeroPage);
  			firstZeroPage = OldSerXidNextPage(firstZeroPage);
  		}
  		slotno = SimpleLruZeroPage(OldSerXidSlruCtl, targetPage);
  	}
  	else
  		slotno = SimpleLruReadPage(OldSerXidSlruCtl, targetPage, true, xid);
  
  	OldSerXidValue(slotno, xid) = minConflictCommitSeqNo;
  	OldSerXidSlruCtl->shared->page_dirty[slotno] = true;
  
  	LWLockRelease(OldSerXidLock);
  }
  
  /*
   * Get the minimum commitSeqNo for any conflict out for the given xid.  For
   * a transaction which exists but has no conflict out, InvalidSerCommitSeqNo
   * will be returned.
   */
  static SerCommitSeqNo
  OldSerXidGetMinConflictCommitSeqNo(TransactionId xid)
  {
  	TransactionId headXid;
  	TransactionId tailXid;
  	SerCommitSeqNo val;
  	int			slotno;
  
  	Assert(TransactionIdIsValid(xid));
  
  	LWLockAcquire(OldSerXidLock, LW_SHARED);
  	headXid = oldSerXidControl->headXid;
  	tailXid = oldSerXidControl->tailXid;
  	LWLockRelease(OldSerXidLock);
  
  	if (!TransactionIdIsValid(headXid))
  		return 0;
  
  	Assert(TransactionIdIsValid(tailXid));
  
  	if (TransactionIdPrecedes(xid, tailXid)
  		|| TransactionIdFollows(xid, headXid))
  		return 0;
  
  	/*
  	 * The following function must be called without holding OldSerXidLock,
  	 * but will return with that lock held, which must then be released.
  	 */
  	slotno = SimpleLruReadPage_ReadOnly(OldSerXidSlruCtl,
  										OldSerXidPage(xid), xid);
  	val = OldSerXidValue(slotno, xid);
  	LWLockRelease(OldSerXidLock);
  	return val;
  }
  
  /*
   * Call this whenever there is a new xmin for active serializable
   * transactions.  We don't need to keep information on transactions which
   * precede that.  InvalidTransactionId means none active, so everything in
   * the SLRU can be discarded.
   */
  static void
  OldSerXidSetActiveSerXmin(TransactionId xid)
  {
  	LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);
  
  	/*
  	 * When no sxacts are active, nothing overlaps, set the xid values to
  	 * invalid to show that there are no valid entries.  Don't clear headPage,
  	 * though.  A new xmin might still land on that page, and we don't want to
  	 * repeatedly zero out the same page.
  	 */
  	if (!TransactionIdIsValid(xid))
  	{
  		oldSerXidControl->tailXid = InvalidTransactionId;
  		oldSerXidControl->headXid = InvalidTransactionId;
  		LWLockRelease(OldSerXidLock);
  		return;
  	}
  
  	/*
  	 * When we're recovering prepared transactions, the global xmin might move
  	 * backwards depending on the order they're recovered. Normally that's not
  	 * OK, but during recovery no serializable transactions will commit, so
  	 * the SLRU is empty and we can get away with it.
  	 */
  	if (RecoveryInProgress())
  	{
  		Assert(oldSerXidControl->headPage < 0);
  		if (!TransactionIdIsValid(oldSerXidControl->tailXid)
  			|| TransactionIdPrecedes(xid, oldSerXidControl->tailXid))
  		{
  			oldSerXidControl->tailXid = xid;
  		}
  		LWLockRelease(OldSerXidLock);
  		return;
  	}
  
  	Assert(!TransactionIdIsValid(oldSerXidControl->tailXid)
  		   || TransactionIdFollows(xid, oldSerXidControl->tailXid));
  
  	oldSerXidControl->tailXid = xid;
  
  	LWLockRelease(OldSerXidLock);
  }
  
  /*
   * Perform a checkpoint --- either during shutdown, or on-the-fly
   *
   * We don't have any data that needs to survive a restart, but this is a
   * convenient place to truncate the SLRU.
   */
  void
  CheckPointPredicate(void)
  {
  	int			tailPage;
  
  	LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);
  
  	/* Exit quickly if the SLRU is currently not in use. */
  	if (oldSerXidControl->headPage < 0)
  	{
  		LWLockRelease(OldSerXidLock);
  		return;
  	}
  
  	if (TransactionIdIsValid(oldSerXidControl->tailXid))
  	{
  		/* We can truncate the SLRU up to the page containing tailXid */
  		tailPage = OldSerXidPage(oldSerXidControl->tailXid);
  	}
  	else
  	{
  		/*
  		 * The SLRU is no longer needed. Truncate to head before we set head
  		 * invalid.
  		 *
  		 * XXX: It's possible that the SLRU is not needed again until XID
  		 * wrap-around has happened, so that the segment containing headPage
  		 * that we leave behind will appear to be new again. In that case it
  		 * won't be removed until XID horizon advances enough to make it
  		 * current again.
  		 */
  		tailPage = oldSerXidControl->headPage;
  		oldSerXidControl->headPage = -1;
  	}
  
  	LWLockRelease(OldSerXidLock);
  
  	/* Truncate away pages that are no longer required */
  	SimpleLruTruncate(OldSerXidSlruCtl, tailPage);
  
  	/*
  	 * Flush dirty SLRU pages to disk
  	 *
  	 * This is not actually necessary from a correctness point of view. We do
  	 * it merely as a debugging aid.
  	 *
  	 * We're doing this after the truncation to avoid writing pages right
  	 * before deleting the file in which they sit, which would be completely
  	 * pointless.
  	 */
  	SimpleLruFlush(OldSerXidSlruCtl, true);
  }
  
  /*------------------------------------------------------------------------*/
  
  /*
   * InitPredicateLocks -- Initialize the predicate locking data structures.
   *
   * This is called from CreateSharedMemoryAndSemaphores(), which see for
   * more comments.  In the normal postmaster case, the shared hash tables
   * are created here.  Backends inherit the pointers
   * to the shared tables via fork().  In the EXEC_BACKEND case, each
   * backend re-executes this code to obtain pointers to the already existing
   * shared hash tables.
   */
  void
  InitPredicateLocks(void)
  {
  	HASHCTL		info;
  	long		max_table_size;
  	Size		requestSize;
  	bool		found;
  
  #ifndef EXEC_BACKEND
  	Assert(!IsUnderPostmaster);
  #endif
  
  	/*
  	 * Compute size of predicate lock target hashtable. Note these
  	 * calculations must agree with PredicateLockShmemSize!
  	 */
  	max_table_size = NPREDICATELOCKTARGETENTS();
  
  	/*
  	 * Allocate hash table for PREDICATELOCKTARGET structs.  This stores
  	 * per-predicate-lock-target information.
  	 */
  	MemSet(&info, 0, sizeof(info));
  	info.keysize = sizeof(PREDICATELOCKTARGETTAG);
  	info.entrysize = sizeof(PREDICATELOCKTARGET);
  	info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
  
  	PredicateLockTargetHash = ShmemInitHash("PREDICATELOCKTARGET hash",
  											max_table_size,
  											max_table_size,
  											&info,
  											HASH_ELEM | HASH_BLOBS |
  											HASH_PARTITION | HASH_FIXED_SIZE);
  
  	/*
  	 * Reserve a dummy entry in the hash table; we use it to make sure there's
  	 * always one entry available when we need to split or combine a page,
  	 * because running out of space there could mean aborting a
  	 * non-serializable transaction.
  	 */
  	if (!IsUnderPostmaster)
  	{
  		(void) hash_search(PredicateLockTargetHash, &ScratchTargetTag,
  						   HASH_ENTER, &found);
  		Assert(!found);
  	}
  
  	/* Pre-calculate the hash and partition lock of the scratch entry */
  	ScratchTargetTagHash = PredicateLockTargetTagHashCode(&ScratchTargetTag);
  	ScratchPartitionLock = PredicateLockHashPartitionLock(ScratchTargetTagHash);
  
  	/*
  	 * Allocate hash table for PREDICATELOCK structs.  This stores per
  	 * xact-lock-of-a-target information.
  	 */
  	MemSet(&info, 0, sizeof(info));
  	info.keysize = sizeof(PREDICATELOCKTAG);
  	info.entrysize = sizeof(PREDICATELOCK);
  	info.hash = predicatelock_hash;
  	info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
  
  	/* Assume an average of 2 xacts per target */
  	max_table_size *= 2;
  
  	PredicateLockHash = ShmemInitHash("PREDICATELOCK hash",
  									  max_table_size,
  									  max_table_size,
  									  &info,
  									  HASH_ELEM | HASH_FUNCTION |
  									  HASH_PARTITION | HASH_FIXED_SIZE);
  
  	/*
  	 * Compute size for serializable transaction hashtable. Note these
  	 * calculations must agree with PredicateLockShmemSize!
  	 */
  	max_table_size = (MaxBackends + max_prepared_xacts);
  
  	/*
  	 * Allocate a list to hold information on transactions participating in
  	 * predicate locking.
  	 *
  	 * Assume an average of 10 predicate locking transactions per backend.
  	 * This allows aggressive cleanup while detail is present before data must
  	 * be summarized for storage in SLRU and the "dummy" transaction.
  	 */
  	max_table_size *= 10;
  
  	PredXact = ShmemInitStruct("PredXactList",
  							   PredXactListDataSize,
  							   &found);
  	Assert(found == IsUnderPostmaster);
  	if (!found)
  	{
  		int			i;
  
  		SHMQueueInit(&PredXact->availableList);
  		SHMQueueInit(&PredXact->activeList);
  		PredXact->SxactGlobalXmin = InvalidTransactionId;
  		PredXact->SxactGlobalXminCount = 0;
  		PredXact->WritableSxactCount = 0;
  		PredXact->LastSxactCommitSeqNo = FirstNormalSerCommitSeqNo - 1;
  		PredXact->CanPartialClearThrough = 0;
  		PredXact->HavePartialClearedThrough = 0;
  		requestSize = mul_size((Size) max_table_size,
  							   PredXactListElementDataSize);
  		PredXact->element = ShmemAlloc(requestSize);
  		/* Add all elements to available list, clean. */
  		memset(PredXact->element, 0, requestSize);
  		for (i = 0; i < max_table_size; i++)
  		{
  			SHMQueueInsertBefore(&(PredXact->availableList),
  								 &(PredXact->element[i].link));
  		}
  		PredXact->OldCommittedSxact = CreatePredXact();
  		SetInvalidVirtualTransactionId(PredXact->OldCommittedSxact->vxid);
  		PredXact->OldCommittedSxact->prepareSeqNo = 0;
  		PredXact->OldCommittedSxact->commitSeqNo = 0;
  		PredXact->OldCommittedSxact->SeqNo.lastCommitBeforeSnapshot = 0;
  		SHMQueueInit(&PredXact->OldCommittedSxact->outConflicts);
  		SHMQueueInit(&PredXact->OldCommittedSxact->inConflicts);
  		SHMQueueInit(&PredXact->OldCommittedSxact->predicateLocks);
  		SHMQueueInit(&PredXact->OldCommittedSxact->finishedLink);
  		SHMQueueInit(&PredXact->OldCommittedSxact->possibleUnsafeConflicts);
  		PredXact->OldCommittedSxact->topXid = InvalidTransactionId;
  		PredXact->OldCommittedSxact->finishedBefore = InvalidTransactionId;
  		PredXact->OldCommittedSxact->xmin = InvalidTransactionId;
  		PredXact->OldCommittedSxact->flags = SXACT_FLAG_COMMITTED;
  		PredXact->OldCommittedSxact->pid = 0;
  	}
  	/* This never changes, so let's keep a local copy. */
  	OldCommittedSxact = PredXact->OldCommittedSxact;
  
  	/*
  	 * Allocate hash table for SERIALIZABLEXID structs.  This stores per-xid
  	 * information for serializable transactions which have accessed data.
  	 */
  	MemSet(&info, 0, sizeof(info));
  	info.keysize = sizeof(SERIALIZABLEXIDTAG);
  	info.entrysize = sizeof(SERIALIZABLEXID);
  
  	SerializableXidHash = ShmemInitHash("SERIALIZABLEXID hash",
  										max_table_size,
  										max_table_size,
  										&info,
  										HASH_ELEM | HASH_BLOBS |
  										HASH_FIXED_SIZE);
  
  	/*
  	 * Allocate space for tracking rw-conflicts in lists attached to the
  	 * transactions.
  	 *
  	 * Assume an average of 5 conflicts per transaction.  Calculations suggest
  	 * that this will prevent resource exhaustion in even the most pessimal
  	 * loads up to max_connections = 200 with all 200 connections pounding the
  	 * database with serializable transactions.  Beyond that, there may be
  	 * occasional transactions canceled when trying to flag conflicts. That's
  	 * probably OK.
  	 */
  	max_table_size *= 5;
  
  	RWConflictPool = ShmemInitStruct("RWConflictPool",
  									 RWConflictPoolHeaderDataSize,
  									 &found);
  	Assert(found == IsUnderPostmaster);
  	if (!found)
  	{
  		int			i;
  
  		SHMQueueInit(&RWConflictPool->availableList);
  		requestSize = mul_size((Size) max_table_size,
  							   RWConflictDataSize);
  		RWConflictPool->element = ShmemAlloc(requestSize);
  		/* Add all elements to available list, clean. */
  		memset(RWConflictPool->element, 0, requestSize);
  		for (i = 0; i < max_table_size; i++)
  		{
  			SHMQueueInsertBefore(&(RWConflictPool->availableList),
  								 &(RWConflictPool->element[i].outLink));
  		}
  	}
  
  	/*
  	 * Create or attach to the header for the list of finished serializable
  	 * transactions.
  	 */
  	FinishedSerializableTransactions = (SHM_QUEUE *)
  		ShmemInitStruct("FinishedSerializableTransactions",
  						sizeof(SHM_QUEUE),
  						&found);
  	Assert(found == IsUnderPostmaster);
  	if (!found)
  		SHMQueueInit(FinishedSerializableTransactions);
  
  	/*
  	 * Initialize the SLRU storage for old committed serializable
  	 * transactions.
  	 */
  	OldSerXidInit();
  }
  
  /*
   * Estimate shared-memory space used for predicate lock table
   */
  Size
  PredicateLockShmemSize(void)
  {
  	Size		size = 0;
  	long		max_table_size;
  
  	/* predicate lock target hash table */
  	max_table_size = NPREDICATELOCKTARGETENTS();
  	size = add_size(size, hash_estimate_size(max_table_size,
  											 sizeof(PREDICATELOCKTARGET)));
  
  	/* predicate lock hash table */
  	max_table_size *= 2;
  	size = add_size(size, hash_estimate_size(max_table_size,
  											 sizeof(PREDICATELOCK)));
  
  	/*
  	 * Since NPREDICATELOCKTARGETENTS is only an estimate, add 10% safety
  	 * margin.
  	 */
  	size = add_size(size, size / 10);
  
  	/* transaction list */
  	max_table_size = MaxBackends + max_prepared_xacts;
  	max_table_size *= 10;
  	size = add_size(size, PredXactListDataSize);
  	size = add_size(size, mul_size((Size) max_table_size,
  								   PredXactListElementDataSize));
  
  	/* transaction xid table */
  	size = add_size(size, hash_estimate_size(max_table_size,
  											 sizeof(SERIALIZABLEXID)));
  
  	/* rw-conflict pool */
  	max_table_size *= 5;
  	size = add_size(size, RWConflictPoolHeaderDataSize);
  	size = add_size(size, mul_size((Size) max_table_size,
  								   RWConflictDataSize));
  
  	/* Head for list of finished serializable transactions. */
  	size = add_size(size, sizeof(SHM_QUEUE));
  
  	/* Shared memory structures for SLRU tracking of old committed xids. */
  	size = add_size(size, sizeof(OldSerXidControlData));
  	size = add_size(size, SimpleLruShmemSize(NUM_OLDSERXID_BUFFERS, 0));
  
  	return size;
  }
  
  
  /*
   * Compute the hash code associated with a PREDICATELOCKTAG.
   *
   * Because we want to use just one set of partition locks for both the
   * PREDICATELOCKTARGET and PREDICATELOCK hash tables, we have to make sure
   * that PREDICATELOCKs fall into the same partition number as their
   * associated PREDICATELOCKTARGETs.  dynahash.c expects the partition number
   * to be the low-order bits of the hash code, and therefore a
   * PREDICATELOCKTAG's hash code must have the same low-order bits as the
   * associated PREDICATELOCKTARGETTAG's hash code.  We achieve this with this
   * specialized hash function.
   */
  static uint32
  predicatelock_hash(const void *key, Size keysize)
  {
  	const PREDICATELOCKTAG *predicatelocktag = (const PREDICATELOCKTAG *) key;
  	uint32		targethash;
  
  	Assert(keysize == sizeof(PREDICATELOCKTAG));
  
  	/* Look into the associated target object, and compute its hash code */
  	targethash = PredicateLockTargetTagHashCode(&predicatelocktag->myTarget->tag);
  
  	return PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash);
  }
  
  
  /*
   * GetPredicateLockStatusData
   *		Return a table containing the internal state of the predicate
   *		lock manager for use in pg_lock_status.
   *
   * Like GetLockStatusData, this function tries to hold the partition LWLocks
   * for as short a time as possible by returning two arrays that simply
   * contain the PREDICATELOCKTARGETTAG and SERIALIZABLEXACT for each lock
   * table entry. Multiple copies of the same PREDICATELOCKTARGETTAG and
   * SERIALIZABLEXACT will likely appear.
   */
  PredicateLockData *
  GetPredicateLockStatusData(void)
  {
  	PredicateLockData *data;
  	int			i;
  	int			els,
  				el;
  	HASH_SEQ_STATUS seqstat;
  	PREDICATELOCK *predlock;
  
  	data = (PredicateLockData *) palloc(sizeof(PredicateLockData));
  
  	/*
  	 * To ensure consistency, take simultaneous locks on all partition locks
  	 * in ascending order, then SerializableXactHashLock.
  	 */
  	for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
  		LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_SHARED);
  	LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  
  	/* Get number of locks and allocate appropriately-sized arrays. */
  	els = hash_get_num_entries(PredicateLockHash);
  	data->nelements = els;
  	data->locktags = (PREDICATELOCKTARGETTAG *)
  		palloc(sizeof(PREDICATELOCKTARGETTAG) * els);
  	data->xacts = (SERIALIZABLEXACT *)
  		palloc(sizeof(SERIALIZABLEXACT) * els);
  
  
  	/* Scan through PredicateLockHash and copy contents */
  	hash_seq_init(&seqstat, PredicateLockHash);
  
  	el = 0;
  
  	while ((predlock = (PREDICATELOCK *) hash_seq_search(&seqstat)))
  	{
  		data->locktags[el] = predlock->tag.myTarget->tag;
  		data->xacts[el] = *predlock->tag.myXact;
  		el++;
  	}
  
  	Assert(el == els);
  
  	/* Release locks in reverse order */
  	LWLockRelease(SerializableXactHashLock);
  	for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
  		LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
  
  	return data;
  }
  
  /*
   * Free up shared memory structures by pushing the oldest sxact (the one at
   * the front of the SummarizeOldestCommittedSxact queue) into summary form.
   * Each call will free exactly one SERIALIZABLEXACT structure and may also
   * free one or more of these structures: SERIALIZABLEXID, PREDICATELOCK,
   * PREDICATELOCKTARGET, RWConflictData.
   */
  static void
  SummarizeOldestCommittedSxact(void)
  {
  	SERIALIZABLEXACT *sxact;
  
  	LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
  
  	/*
  	 * This function is only called if there are no sxact slots available.
  	 * Some of them must belong to old, already-finished transactions, so
  	 * there should be something in FinishedSerializableTransactions list that
  	 * we can summarize. However, there's a race condition: while we were not
  	 * holding any locks, a transaction might have ended and cleaned up all
  	 * the finished sxact entries already, freeing up their sxact slots. In
  	 * that case, we have nothing to do here. The caller will find one of the
  	 * slots released by the other backend when it retries.
  	 */
  	if (SHMQueueEmpty(FinishedSerializableTransactions))
  	{
  		LWLockRelease(SerializableFinishedListLock);
  		return;
  	}
  
  	/*
  	 * Grab the first sxact off the finished list -- this will be the earliest
  	 * commit.  Remove it from the list.
  	 */
  	sxact = (SERIALIZABLEXACT *)
  		SHMQueueNext(FinishedSerializableTransactions,
  					 FinishedSerializableTransactions,
  					 offsetof(SERIALIZABLEXACT, finishedLink));
  	SHMQueueDelete(&(sxact->finishedLink));
  
  	/* Add to SLRU summary information. */
  	if (TransactionIdIsValid(sxact->topXid) && !SxactIsReadOnly(sxact))
  		OldSerXidAdd(sxact->topXid, SxactHasConflictOut(sxact)
  					 ? sxact->SeqNo.earliestOutConflictCommit : InvalidSerCommitSeqNo);
  
  	/* Summarize and release the detail. */
  	ReleaseOneSerializableXact(sxact, false, true);
  
  	LWLockRelease(SerializableFinishedListLock);
  }
  
  /*
   * GetSafeSnapshot
   *		Obtain and register a snapshot for a READ ONLY DEFERRABLE
   *		transaction. Ensures that the snapshot is "safe", i.e. a
   *		read-only transaction running on it can execute serializably
   *		without further checks. This requires waiting for concurrent
   *		transactions to complete, and retrying with a new snapshot if
   *		one of them could possibly create a conflict.
   *
   *		As with GetSerializableTransactionSnapshot (which this is a subroutine
   *		for), the passed-in Snapshot pointer should reference a static data
   *		area that can safely be passed to GetSnapshotData.
   */
  static Snapshot
  GetSafeSnapshot(Snapshot origSnapshot)
  {
  	Snapshot	snapshot;
  
  	Assert(XactReadOnly && XactDeferrable);
  
  	while (true)
  	{
  		/*
  		 * GetSerializableTransactionSnapshotInt is going to call
  		 * GetSnapshotData, so we need to provide it the static snapshot area
  		 * our caller passed to us.  The pointer returned is actually the same
  		 * one passed to it, but we avoid assuming that here.
  		 */
  		snapshot = GetSerializableTransactionSnapshotInt(origSnapshot,
  														 NULL, InvalidPid);
  
  		if (MySerializableXact == InvalidSerializableXact)
  			return snapshot;	/* no concurrent r/w xacts; it's safe */
  
  		LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  		/*
  		 * Wait for concurrent transactions to finish. Stop early if one of
  		 * them marked us as conflicted.
  		 */
  		MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;
  		while (!(SHMQueueEmpty(&MySerializableXact->possibleUnsafeConflicts) ||
  				 SxactIsROUnsafe(MySerializableXact)))
  		{
  			LWLockRelease(SerializableXactHashLock);
  			ProcWaitForSignal(WAIT_EVENT_SAFE_SNAPSHOT);
  			LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  		}
  		MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;
  
  		if (!SxactIsROUnsafe(MySerializableXact))
  		{
  			LWLockRelease(SerializableXactHashLock);
  			break;				/* success */
  		}
  
  		LWLockRelease(SerializableXactHashLock);
  
  		/* else, need to retry... */
  		ereport(DEBUG2,
  				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  				 errmsg("deferrable snapshot was unsafe; trying a new one")));
  		ReleasePredicateLocks(false);
  	}
  
  	/*
  	 * Now we have a safe snapshot, so we don't need to do any further checks.
  	 */
  	Assert(SxactIsROSafe(MySerializableXact));
  	ReleasePredicateLocks(false);
  
  	return snapshot;
  }
  
  /*
   * GetSafeSnapshotBlockingPids
   *		If the specified process is currently blocked in GetSafeSnapshot,
   *		write the process IDs of all processes that it is blocked by
   *		into the caller-supplied buffer output[].  The list is truncated at
   *		output_size, and the number of PIDs written into the buffer is
   *		returned.  Returns zero if the given PID is not currently blocked
   *		in GetSafeSnapshot.
   */
  int
  GetSafeSnapshotBlockingPids(int blocked_pid, int *output, int output_size)
  {
  	int			num_written = 0;
  	SERIALIZABLEXACT *sxact;
  
  	LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  
  	/* Find blocked_pid's SERIALIZABLEXACT by linear search. */
  	for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))
  	{
  		if (sxact->pid == blocked_pid)
  			break;
  	}
  
  	/* Did we find it, and is it currently waiting in GetSafeSnapshot? */
  	if (sxact != NULL && SxactIsDeferrableWaiting(sxact))
  	{
  		RWConflict	possibleUnsafeConflict;
  
  		/* Traverse the list of possible unsafe conflicts collecting PIDs. */
  		possibleUnsafeConflict = (RWConflict)
  			SHMQueueNext(&sxact->possibleUnsafeConflicts,
  						 &sxact->possibleUnsafeConflicts,
  						 offsetof(RWConflictData, inLink));
  
  		while (possibleUnsafeConflict != NULL && num_written < output_size)
  		{
  			output[num_written++] = possibleUnsafeConflict->sxactOut->pid;
  			possibleUnsafeConflict = (RWConflict)
  				SHMQueueNext(&sxact->possibleUnsafeConflicts,
  							 &possibleUnsafeConflict->inLink,
  							 offsetof(RWConflictData, inLink));
  		}
  	}
  
  	LWLockRelease(SerializableXactHashLock);
  
  	return num_written;
  }
  
  /*
   * Acquire a snapshot that can be used for the current transaction.
   *
   * Make sure we have a SERIALIZABLEXACT reference in MySerializableXact.
   * It should be current for this process and be contained in PredXact.
   *
   * The passed-in Snapshot pointer should reference a static data area that
   * can safely be passed to GetSnapshotData.  The return value is actually
   * always this same pointer; no new snapshot data structure is allocated
   * within this function.
   */
  Snapshot
  GetSerializableTransactionSnapshot(Snapshot snapshot)
  {
  	Assert(IsolationIsSerializable());
  
  	/*
  	 * Can't use serializable mode while recovery is still active, as it is,
  	 * for example, on a hot standby.  We could get here despite the check in
  	 * check_XactIsoLevel() if default_transaction_isolation is set to
  	 * serializable, so phrase the hint accordingly.
  	 */
  	if (RecoveryInProgress())
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot use serializable mode in a hot standby"),
  				 errdetail("\"default_transaction_isolation\" is set to \"serializable\"."),
  				 errhint("You can use \"SET default_transaction_isolation = 'repeatable read'\" to change the default.")));
  
  	/*
  	 * A special optimization is available for SERIALIZABLE READ ONLY
  	 * DEFERRABLE transactions -- we can wait for a suitable snapshot and
  	 * thereby avoid all SSI overhead once it's running.
  	 */
  	if (XactReadOnly && XactDeferrable)
  		return GetSafeSnapshot(snapshot);
  
  	return GetSerializableTransactionSnapshotInt(snapshot,
  												 NULL, InvalidPid);
  }
  
  /*
   * Import a snapshot to be used for the current transaction.
   *
   * This is nearly the same as GetSerializableTransactionSnapshot, except that
   * we don't take a new snapshot, but rather use the data we're handed.
   *
   * The caller must have verified that the snapshot came from a serializable
   * transaction; and if we're read-write, the source transaction must not be
   * read-only.
   */
  void
  SetSerializableTransactionSnapshot(Snapshot snapshot,
  								   VirtualTransactionId *sourcevxid,
  								   int sourcepid)
  {
  	Assert(IsolationIsSerializable());
  
  	/*
  	 * We do not allow SERIALIZABLE READ ONLY DEFERRABLE transactions to
  	 * import snapshots, since there's no way to wait for a safe snapshot when
  	 * we're using the snap we're told to.  (XXX instead of throwing an error,
  	 * we could just ignore the XactDeferrable flag?)
  	 */
  	if (XactReadOnly && XactDeferrable)
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("a snapshot-importing transaction must not be READ ONLY DEFERRABLE")));
  
  	(void) GetSerializableTransactionSnapshotInt(snapshot, sourcevxid,
  												 sourcepid);
  }
  
  /*
   * Guts of GetSerializableTransactionSnapshot
   *
   * If sourcexid is valid, this is actually an import operation and we should
   * skip calling GetSnapshotData, because the snapshot contents are already
   * loaded up.  HOWEVER: to avoid race conditions, we must check that the
   * source xact is still running after we acquire SerializableXactHashLock.
   * We do that by calling ProcArrayInstallImportedXmin.
   */
  static Snapshot
  GetSerializableTransactionSnapshotInt(Snapshot snapshot,
  									  VirtualTransactionId *sourcevxid,
  									  int sourcepid)
  {
  	PGPROC	   *proc;
  	VirtualTransactionId vxid;
  	SERIALIZABLEXACT *sxact,
  			   *othersxact;
  	HASHCTL		hash_ctl;
  
  	/* We only do this for serializable transactions.  Once. */
  	Assert(MySerializableXact == InvalidSerializableXact);
  
  	Assert(!RecoveryInProgress());
  
  	/*
  	 * Since all parts of a serializable transaction must use the same
  	 * snapshot, it is too late to establish one after a parallel operation
  	 * has begun.
  	 */
  	if (IsInParallelMode())
  		elog(ERROR, "cannot establish serializable snapshot during a parallel operation");
  
  	proc = MyProc;
  	Assert(proc != NULL);
  	GET_VXID_FROM_PGPROC(vxid, *proc);
  
  	/*
  	 * First we get the sxact structure, which may involve looping and access
  	 * to the "finished" list to free a structure for use.
  	 *
  	 * We must hold SerializableXactHashLock when taking/checking the snapshot
  	 * to avoid race conditions, for much the same reasons that
  	 * GetSnapshotData takes the ProcArrayLock.  Since we might have to
  	 * release SerializableXactHashLock to call SummarizeOldestCommittedSxact,
  	 * this means we have to create the sxact first, which is a bit annoying
  	 * (in particular, an elog(ERROR) in procarray.c would cause us to leak
  	 * the sxact).  Consider refactoring to avoid this.
  	 */
  #ifdef TEST_OLDSERXID
  	SummarizeOldestCommittedSxact();
  #endif
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  	do
  	{
  		sxact = CreatePredXact();
  		/* If null, push out committed sxact to SLRU summary & retry. */
  		if (!sxact)
  		{
  			LWLockRelease(SerializableXactHashLock);
  			SummarizeOldestCommittedSxact();
  			LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  		}
  	} while (!sxact);
  
  	/* Get the snapshot, or check that it's safe to use */
  	if (!sourcevxid)
  		snapshot = GetSnapshotData(snapshot);
  	else if (!ProcArrayInstallImportedXmin(snapshot->xmin, sourcevxid))
  	{
  		ReleasePredXact(sxact);
  		LWLockRelease(SerializableXactHashLock);
  		ereport(ERROR,
  				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
  				 errmsg("could not import the requested snapshot"),
  				 errdetail("The source process with PID %d is not running anymore.",
  						   sourcepid)));
  	}
  
  	/*
  	 * If there are no serializable transactions which are not read-only, we
  	 * can "opt out" of predicate locking and conflict checking for a
  	 * read-only transaction.
  	 *
  	 * The reason this is safe is that a read-only transaction can only become
  	 * part of a dangerous structure if it overlaps a writable transaction
  	 * which in turn overlaps a writable transaction which committed before
  	 * the read-only transaction started.  A new writable transaction can
  	 * overlap this one, but it can't meet the other condition of overlapping
  	 * a transaction which committed before this one started.
  	 */
  	if (XactReadOnly && PredXact->WritableSxactCount == 0)
  	{
  		ReleasePredXact(sxact);
  		LWLockRelease(SerializableXactHashLock);
  		return snapshot;
  	}
  
  	/* Maintain serializable global xmin info. */
  	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
  	{
  		Assert(PredXact->SxactGlobalXminCount == 0);
  		PredXact->SxactGlobalXmin = snapshot->xmin;
  		PredXact->SxactGlobalXminCount = 1;
  		OldSerXidSetActiveSerXmin(snapshot->xmin);
  	}
  	else if (TransactionIdEquals(snapshot->xmin, PredXact->SxactGlobalXmin))
  	{
  		Assert(PredXact->SxactGlobalXminCount > 0);
  		PredXact->SxactGlobalXminCount++;
  	}
  	else
  	{
  		Assert(TransactionIdFollows(snapshot->xmin, PredXact->SxactGlobalXmin));
  	}
  
  	/* Initialize the structure. */
  	sxact->vxid = vxid;
  	sxact->SeqNo.lastCommitBeforeSnapshot = PredXact->LastSxactCommitSeqNo;
  	sxact->prepareSeqNo = InvalidSerCommitSeqNo;
  	sxact->commitSeqNo = InvalidSerCommitSeqNo;
  	SHMQueueInit(&(sxact->outConflicts));
  	SHMQueueInit(&(sxact->inConflicts));
  	SHMQueueInit(&(sxact->possibleUnsafeConflicts));
  	sxact->topXid = GetTopTransactionIdIfAny();
  	sxact->finishedBefore = InvalidTransactionId;
  	sxact->xmin = snapshot->xmin;
  	sxact->pid = MyProcPid;
  	SHMQueueInit(&(sxact->predicateLocks));
  	SHMQueueElemInit(&(sxact->finishedLink));
  	sxact->flags = 0;
  	if (XactReadOnly)
  	{
  		sxact->flags |= SXACT_FLAG_READ_ONLY;
  
  		/*
  		 * Register all concurrent r/w transactions as possible conflicts; if
  		 * all of them commit without any outgoing conflicts to earlier
  		 * transactions then this snapshot can be deemed safe (and we can run
  		 * without tracking predicate locks).
  		 */
  		for (othersxact = FirstPredXact();
  			 othersxact != NULL;
  			 othersxact = NextPredXact(othersxact))
  		{
  			if (!SxactIsCommitted(othersxact)
  				&& !SxactIsDoomed(othersxact)
  				&& !SxactIsReadOnly(othersxact))
  			{
  				SetPossibleUnsafeConflict(sxact, othersxact);
  			}
  		}
  	}
  	else
  	{
  		++(PredXact->WritableSxactCount);
  		Assert(PredXact->WritableSxactCount <=
  			   (MaxBackends + max_prepared_xacts));
  	}
  
  	MySerializableXact = sxact;
  	MyXactDidWrite = false;		/* haven't written anything yet */
  
  	LWLockRelease(SerializableXactHashLock);
  
  	/* Initialize the backend-local hash table of parent locks */
  	Assert(LocalPredicateLockHash == NULL);
  	MemSet(&hash_ctl, 0, sizeof(hash_ctl));
  	hash_ctl.keysize = sizeof(PREDICATELOCKTARGETTAG);
  	hash_ctl.entrysize = sizeof(LOCALPREDICATELOCK);
  	LocalPredicateLockHash = hash_create("Local predicate lock",
  										 max_predicate_locks_per_xact,
  										 &hash_ctl,
  										 HASH_ELEM | HASH_BLOBS);
  
  	return snapshot;
  }
  
  /*
   * Register the top level XID in SerializableXidHash.
   * Also store it for easy reference in MySerializableXact.
   */
  void
  RegisterPredicateLockingXid(TransactionId xid)
  {
  	SERIALIZABLEXIDTAG sxidtag;
  	SERIALIZABLEXID *sxid;
  	bool		found;
  
  	/*
  	 * If we're not tracking predicate lock data for this transaction, we
  	 * should ignore the request and return quickly.
  	 */
  	if (MySerializableXact == InvalidSerializableXact)
  		return;
  
  	/* We should have a valid XID and be at the top level. */
  	Assert(TransactionIdIsValid(xid));
  
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	/* This should only be done once per transaction. */
  	Assert(MySerializableXact->topXid == InvalidTransactionId);
  
  	MySerializableXact->topXid = xid;
  
  	sxidtag.xid = xid;
  	sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
  										   &sxidtag,
  										   HASH_ENTER, &found);
  	Assert(!found);
  
  	/* Initialize the structure. */
  	sxid->myXact = MySerializableXact;
  	LWLockRelease(SerializableXactHashLock);
  }
  
  
  /*
   * Check whether there are any predicate locks held by any transaction
   * for the page at the given block number.
   *
   * Note that the transaction may be completed but not yet subject to
   * cleanup due to overlapping serializable transactions.  This must
   * return valid information regardless of transaction isolation level.
   *
   * Also note that this doesn't check for a conflicting relation lock,
   * just a lock specifically on the given page.
   *
   * One use is to support proper behavior during GiST index vacuum.
   */
  bool
  PageIsPredicateLocked(Relation relation, BlockNumber blkno)
  {
  	PREDICATELOCKTARGETTAG targettag;
  	uint32		targettaghash;
  	LWLock	   *partitionLock;
  	PREDICATELOCKTARGET *target;
  
  	SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
  									relation->rd_node.dbNode,
  									relation->rd_id,
  									blkno);
  
  	targettaghash = PredicateLockTargetTagHashCode(&targettag);
  	partitionLock = PredicateLockHashPartitionLock(targettaghash);
  	LWLockAcquire(partitionLock, LW_SHARED);
  	target = (PREDICATELOCKTARGET *)
  		hash_search_with_hash_value(PredicateLockTargetHash,
  									&targettag, targettaghash,
  									HASH_FIND, NULL);
  	LWLockRelease(partitionLock);
  
  	return (target != NULL);
  }
  
  
  /*
   * Check whether a particular lock is held by this transaction.
   *
   * Important note: this function may return false even if the lock is
   * being held, because it uses the local lock table which is not
   * updated if another transaction modifies our lock list (e.g. to
   * split an index page). It can also return true when a coarser
   * granularity lock that covers this target is being held. Be careful
   * to only use this function in circumstances where such errors are
   * acceptable!
   */
  static bool
  PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag)
  {
  	LOCALPREDICATELOCK *lock;
  
  	/* check local hash table */
  	lock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
  											  targettag,
  											  HASH_FIND, NULL);
  
  	if (!lock)
  		return false;
  
  	/*
  	 * Found entry in the table, but still need to check whether it's actually
  	 * held -- it could just be a parent of some held lock.
  	 */
  	return lock->held;
  }
  
  /*
   * Return the parent lock tag in the lock hierarchy: the next coarser
   * lock that covers the provided tag.
   *
   * Returns true and sets *parent to the parent tag if one exists,
   * returns false if none exists.
   */
  static bool
  GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent)
  {
  	switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
  	{
  		case PREDLOCKTAG_RELATION:
  			/* relation locks have no parent lock */
  			return false;
  
  		case PREDLOCKTAG_PAGE:
  			/* parent lock is relation lock */
  			SET_PREDICATELOCKTARGETTAG_RELATION(*parent,
  												GET_PREDICATELOCKTARGETTAG_DB(*tag),
  												GET_PREDICATELOCKTARGETTAG_RELATION(*tag));
  
  			return true;
  
  		case PREDLOCKTAG_TUPLE:
  			/* parent lock is page lock */
  			SET_PREDICATELOCKTARGETTAG_PAGE(*parent,
  											GET_PREDICATELOCKTARGETTAG_DB(*tag),
  											GET_PREDICATELOCKTARGETTAG_RELATION(*tag),
  											GET_PREDICATELOCKTARGETTAG_PAGE(*tag));
  			return true;
  	}
  
  	/* not reachable */
  	Assert(false);
  	return false;
  }
  
  /*
   * Check whether the lock we are considering is already covered by a
   * coarser lock for our transaction.
   *
   * Like PredicateLockExists, this function might return a false
   * negative, but it will never return a false positive.
   */
  static bool
  CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag)
  {
  	PREDICATELOCKTARGETTAG targettag,
  				parenttag;
  
  	targettag = *newtargettag;
  
  	/* check parents iteratively until no more */
  	while (GetParentPredicateLockTag(&targettag, &parenttag))
  	{
  		targettag = parenttag;
  		if (PredicateLockExists(&targettag))
  			return true;
  	}
  
  	/* no more parents to check; lock is not covered */
  	return false;
  }
  
  /*
   * Remove the dummy entry from the predicate lock target hash, to free up some
   * scratch space. The caller must be holding SerializablePredicateLockListLock,
   * and must restore the entry with RestoreScratchTarget() before releasing the
   * lock.
   *
   * If lockheld is true, the caller is already holding the partition lock
   * of the partition containing the scratch entry.
   */
  static void
  RemoveScratchTarget(bool lockheld)
  {
  	bool		found;
  
  	Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
  
  	if (!lockheld)
  		LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
  	hash_search_with_hash_value(PredicateLockTargetHash,
  								&ScratchTargetTag,
  								ScratchTargetTagHash,
  								HASH_REMOVE, &found);
  	Assert(found);
  	if (!lockheld)
  		LWLockRelease(ScratchPartitionLock);
  }
  
  /*
   * Re-insert the dummy entry in predicate lock target hash.
   */
  static void
  RestoreScratchTarget(bool lockheld)
  {
  	bool		found;
  
  	Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
  
  	if (!lockheld)
  		LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
  	hash_search_with_hash_value(PredicateLockTargetHash,
  								&ScratchTargetTag,
  								ScratchTargetTagHash,
  								HASH_ENTER, &found);
  	Assert(!found);
  	if (!lockheld)
  		LWLockRelease(ScratchPartitionLock);
  }
  
  /*
   * Check whether the list of related predicate locks is empty for a
   * predicate lock target, and remove the target if it is.
   */
  static void
  RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target, uint32 targettaghash)
  {
  	PREDICATELOCKTARGET *rmtarget PG_USED_FOR_ASSERTS_ONLY;
  
  	Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
  
  	/* Can't remove it until no locks at this target. */
  	if (!SHMQueueEmpty(&target->predicateLocks))
  		return;
  
  	/* Actually remove the target. */
  	rmtarget = hash_search_with_hash_value(PredicateLockTargetHash,
  										   &target->tag,
  										   targettaghash,
  										   HASH_REMOVE, NULL);
  	Assert(rmtarget == target);
  }
  
  /*
   * Delete child target locks owned by this process.
   * This implementation is assuming that the usage of each target tag field
   * is uniform.  No need to make this hard if we don't have to.
   *
   * We aren't acquiring lightweight locks for the predicate lock or lock
   * target structures associated with this transaction unless we're going
   * to modify them, because no other process is permitted to modify our
   * locks.
   */
  static void
  DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag)
  {
  	SERIALIZABLEXACT *sxact;
  	PREDICATELOCK *predlock;
  
  	LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  	sxact = MySerializableXact;
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&(sxact->predicateLocks),
  					 &(sxact->predicateLocks),
  					 offsetof(PREDICATELOCK, xactLink));
  	while (predlock)
  	{
  		SHM_QUEUE  *predlocksxactlink;
  		PREDICATELOCK *nextpredlock;
  		PREDICATELOCKTAG oldlocktag;
  		PREDICATELOCKTARGET *oldtarget;
  		PREDICATELOCKTARGETTAG oldtargettag;
  
  		predlocksxactlink = &(predlock->xactLink);
  		nextpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(sxact->predicateLocks),
  						 predlocksxactlink,
  						 offsetof(PREDICATELOCK, xactLink));
  
  		oldlocktag = predlock->tag;
  		Assert(oldlocktag.myXact == sxact);
  		oldtarget = oldlocktag.myTarget;
  		oldtargettag = oldtarget->tag;
  
  		if (TargetTagIsCoveredBy(oldtargettag, *newtargettag))
  		{
  			uint32		oldtargettaghash;
  			LWLock	   *partitionLock;
  			PREDICATELOCK *rmpredlock PG_USED_FOR_ASSERTS_ONLY;
  
  			oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
  			partitionLock = PredicateLockHashPartitionLock(oldtargettaghash);
  
  			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
  
  			SHMQueueDelete(predlocksxactlink);
  			SHMQueueDelete(&(predlock->targetLink));
  			rmpredlock = hash_search_with_hash_value
  				(PredicateLockHash,
  				 &oldlocktag,
  				 PredicateLockHashCodeFromTargetHashCode(&oldlocktag,
  														 oldtargettaghash),
  				 HASH_REMOVE, NULL);
  			Assert(rmpredlock == predlock);
  
  			RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);
  
  			LWLockRelease(partitionLock);
  
  			DecrementParentLocks(&oldtargettag);
  		}
  
  		predlock = nextpredlock;
  	}
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  /*
   * Returns the promotion limit for a given predicate lock target.  This is the
   * max number of descendant locks allowed before promoting to the specified
   * tag. Note that the limit includes non-direct descendants (e.g., both tuples
   * and pages for a relation lock).
   *
   * Currently the default limit is 2 for a page lock, and half of the value of
   * max_pred_locks_per_transaction - 1 for a relation lock, to match behavior
   * of earlier releases when upgrading.
   *
   * TODO SSI: We should probably add additional GUCs to allow a maximum ratio
   * of page and tuple locks based on the pages in a relation, and the maximum
   * ratio of tuple locks to tuples in a page.  This would provide more
   * generally "balanced" allocation of locks to where they are most useful,
   * while still allowing the absolute numbers to prevent one relation from
   * tying up all predicate lock resources.
   */
  static int
  MaxPredicateChildLocks(const PREDICATELOCKTARGETTAG *tag)
  {
  	switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
  	{
  		case PREDLOCKTAG_RELATION:
  			return max_predicate_locks_per_relation < 0
  				? (max_predicate_locks_per_xact
  				   / (-max_predicate_locks_per_relation)) - 1
  				: max_predicate_locks_per_relation;
  
  		case PREDLOCKTAG_PAGE:
  			return max_predicate_locks_per_page;
  
  		case PREDLOCKTAG_TUPLE:
  
  			/*
  			 * not reachable: nothing is finer-granularity than a tuple, so we
  			 * should never try to promote to it.
  			 */
  			Assert(false);
  			return 0;
  	}
  
  	/* not reachable */
  	Assert(false);
  	return 0;
  }
  
  /*
   * For all ancestors of a newly-acquired predicate lock, increment
   * their child count in the parent hash table. If any of them have
   * more descendants than their promotion threshold, acquire the
   * coarsest such lock.
   *
   * Returns true if a parent lock was acquired and false otherwise.
   */
  static bool
  CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag)
  {
  	PREDICATELOCKTARGETTAG targettag,
  				nexttag,
  				promotiontag;
  	LOCALPREDICATELOCK *parentlock;
  	bool		found,
  				promote;
  
  	promote = false;
  
  	targettag = *reqtag;
  
  	/* check parents iteratively */
  	while (GetParentPredicateLockTag(&targettag, &nexttag))
  	{
  		targettag = nexttag;
  		parentlock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
  														&targettag,
  														HASH_ENTER,
  														&found);
  		if (!found)
  		{
  			parentlock->held = false;
  			parentlock->childLocks = 1;
  		}
  		else
  			parentlock->childLocks++;
  
  		if (parentlock->childLocks >
  			MaxPredicateChildLocks(&targettag))
  		{
  			/*
  			 * We should promote to this parent lock. Continue to check its
  			 * ancestors, however, both to get their child counts right and to
  			 * check whether we should just go ahead and promote to one of
  			 * them.
  			 */
  			promotiontag = targettag;
  			promote = true;
  		}
  	}
  
  	if (promote)
  	{
  		/* acquire coarsest ancestor eligible for promotion */
  		PredicateLockAcquire(&promotiontag);
  		return true;
  	}
  	else
  		return false;
  }
  
  /*
   * When releasing a lock, decrement the child count on all ancestor
   * locks.
   *
   * This is called only when releasing a lock via
   * DeleteChildTargetLocks (i.e. when a lock becomes redundant because
   * we've acquired its parent, possibly due to promotion) or when a new
   * MVCC write lock makes the predicate lock unnecessary. There's no
   * point in calling it when locks are released at transaction end, as
   * this information is no longer needed.
   */
  static void
  DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag)
  {
  	PREDICATELOCKTARGETTAG parenttag,
  				nexttag;
  
  	parenttag = *targettag;
  
  	while (GetParentPredicateLockTag(&parenttag, &nexttag))
  	{
  		uint32		targettaghash;
  		LOCALPREDICATELOCK *parentlock,
  				   *rmlock PG_USED_FOR_ASSERTS_ONLY;
  
  		parenttag = nexttag;
  		targettaghash = PredicateLockTargetTagHashCode(&parenttag);
  		parentlock = (LOCALPREDICATELOCK *)
  			hash_search_with_hash_value(LocalPredicateLockHash,
  										&parenttag, targettaghash,
  										HASH_FIND, NULL);
  
  		/*
  		 * There's a small chance the parent lock doesn't exist in the lock
  		 * table. This can happen if we prematurely removed it because an
  		 * index split caused the child refcount to be off.
  		 */
  		if (parentlock == NULL)
  			continue;
  
  		parentlock->childLocks--;
  
  		/*
  		 * Under similar circumstances the parent lock's refcount might be
  		 * zero. This only happens if we're holding that lock (otherwise we
  		 * would have removed the entry).
  		 */
  		if (parentlock->childLocks < 0)
  		{
  			Assert(parentlock->held);
  			parentlock->childLocks = 0;
  		}
  
  		if ((parentlock->childLocks == 0) && (!parentlock->held))
  		{
  			rmlock = (LOCALPREDICATELOCK *)
  				hash_search_with_hash_value(LocalPredicateLockHash,
  											&parenttag, targettaghash,
  											HASH_REMOVE, NULL);
  			Assert(rmlock == parentlock);
  		}
  	}
  }
  
  /*
   * Indicate that a predicate lock on the given target is held by the
   * specified transaction. Has no effect if the lock is already held.
   *
   * This updates the lock table and the sxact's lock list, and creates
   * the lock target if necessary, but does *not* do anything related to
   * granularity promotion or the local lock table. See
   * PredicateLockAcquire for that.
   */
  static void
  CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
  					uint32 targettaghash,
  					SERIALIZABLEXACT *sxact)
  {
  	PREDICATELOCKTARGET *target;
  	PREDICATELOCKTAG locktag;
  	PREDICATELOCK *lock;
  	LWLock	   *partitionLock;
  	bool		found;
  
  	partitionLock = PredicateLockHashPartitionLock(targettaghash);
  
  	LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  	LWLockAcquire(partitionLock, LW_EXCLUSIVE);
  
  	/* Make sure that the target is represented. */
  	target = (PREDICATELOCKTARGET *)
  		hash_search_with_hash_value(PredicateLockTargetHash,
  									targettag, targettaghash,
  									HASH_ENTER_NULL, &found);
  	if (!target)
  		ereport(ERROR,
  				(errcode(ERRCODE_OUT_OF_MEMORY),
  				 errmsg("out of shared memory"),
  				 errhint("You might need to increase max_pred_locks_per_transaction.")));
  	if (!found)
  		SHMQueueInit(&(target->predicateLocks));
  
  	/* We've got the sxact and target, make sure they're joined. */
  	locktag.myTarget = target;
  	locktag.myXact = sxact;
  	lock = (PREDICATELOCK *)
  		hash_search_with_hash_value(PredicateLockHash, &locktag,
  									PredicateLockHashCodeFromTargetHashCode(&locktag, targettaghash),
  									HASH_ENTER_NULL, &found);
  	if (!lock)
  		ereport(ERROR,
  				(errcode(ERRCODE_OUT_OF_MEMORY),
  				 errmsg("out of shared memory"),
  				 errhint("You might need to increase max_pred_locks_per_transaction.")));
  
  	if (!found)
  	{
  		SHMQueueInsertBefore(&(target->predicateLocks), &(lock->targetLink));
  		SHMQueueInsertBefore(&(sxact->predicateLocks),
  							 &(lock->xactLink));
  		lock->commitSeqNo = InvalidSerCommitSeqNo;
  	}
  
  	LWLockRelease(partitionLock);
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  /*
   * Acquire a predicate lock on the specified target for the current
   * connection if not already held. This updates the local lock table
   * and uses it to implement granularity promotion. It will consolidate
   * multiple locks into a coarser lock if warranted, and will release
   * any finer-grained locks covered by the new one.
   */
  static void
  PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag)
  {
  	uint32		targettaghash;
  	bool		found;
  	LOCALPREDICATELOCK *locallock;
  
  	/* Do we have the lock already, or a covering lock? */
  	if (PredicateLockExists(targettag))
  		return;
  
  	if (CoarserLockCovers(targettag))
  		return;
  
  	/* the same hash and LW lock apply to the lock target and the local lock. */
  	targettaghash = PredicateLockTargetTagHashCode(targettag);
  
  	/* Acquire lock in local table */
  	locallock = (LOCALPREDICATELOCK *)
  		hash_search_with_hash_value(LocalPredicateLockHash,
  									targettag, targettaghash,
  									HASH_ENTER, &found);
  	locallock->held = true;
  	if (!found)
  		locallock->childLocks = 0;
  
  	/* Actually create the lock */
  	CreatePredicateLock(targettag, targettaghash, MySerializableXact);
  
  	/*
  	 * Lock has been acquired. Check whether it should be promoted to a
  	 * coarser granularity, or whether there are finer-granularity locks to
  	 * clean up.
  	 */
  	if (CheckAndPromotePredicateLockRequest(targettag))
  	{
  		/*
  		 * Lock request was promoted to a coarser-granularity lock, and that
  		 * lock was acquired. It will delete this lock and any of its
  		 * children, so we're done.
  		 */
  	}
  	else
  	{
  		/* Clean up any finer-granularity locks */
  		if (GET_PREDICATELOCKTARGETTAG_TYPE(*targettag) != PREDLOCKTAG_TUPLE)
  			DeleteChildTargetLocks(targettag);
  	}
  }
  
  
  /*
   *		PredicateLockRelation
   *
   * Gets a predicate lock at the relation level.
   * Skip if not in full serializable transaction isolation level.
   * Skip if this is a temporary table.
   * Clear any finer-grained predicate locks this session has on the relation.
   */
  void
  PredicateLockRelation(Relation relation, Snapshot snapshot)
  {
  	PREDICATELOCKTARGETTAG tag;
  
  	if (!SerializationNeededForRead(relation, snapshot))
  		return;
  
  	SET_PREDICATELOCKTARGETTAG_RELATION(tag,
  										relation->rd_node.dbNode,
  										relation->rd_id);
  	PredicateLockAcquire(&tag);
  }
  
  /*
   *		PredicateLockPage
   *
   * Gets a predicate lock at the page level.
   * Skip if not in full serializable transaction isolation level.
   * Skip if this is a temporary table.
   * Skip if a coarser predicate lock already covers this page.
   * Clear any finer-grained predicate locks this session has on the relation.
   */
  void
  PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot)
  {
  	PREDICATELOCKTARGETTAG tag;
  
  	if (!SerializationNeededForRead(relation, snapshot))
  		return;
  
  	SET_PREDICATELOCKTARGETTAG_PAGE(tag,
  									relation->rd_node.dbNode,
  									relation->rd_id,
  									blkno);
  	PredicateLockAcquire(&tag);
  }
  
  /*
   *		PredicateLockTuple
   *
   * Gets a predicate lock at the tuple level.
   * Skip if not in full serializable transaction isolation level.
   * Skip if this is a temporary table.
   */
  void
  PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
  {
  	PREDICATELOCKTARGETTAG tag;
  	ItemPointer tid;
  	TransactionId targetxmin;
  
  	if (!SerializationNeededForRead(relation, snapshot))
  		return;
  
  	/*
  	 * If it's a heap tuple, return if this xact wrote it.
  	 */
  	if (relation->rd_index == NULL)
  	{
  		TransactionId myxid;
  
  		targetxmin = HeapTupleHeaderGetXmin(tuple->t_data);
  
  		myxid = GetTopTransactionIdIfAny();
  		if (TransactionIdIsValid(myxid))
  		{
  			if (TransactionIdFollowsOrEquals(targetxmin, TransactionXmin))
  			{
  				TransactionId xid = SubTransGetTopmostTransaction(targetxmin);
  
  				if (TransactionIdEquals(xid, myxid))
  				{
  					/* We wrote it; we already have a write lock. */
  					return;
  				}
  			}
  		}
  	}
  
  	/*
  	 * Do quick-but-not-definitive test for a relation lock first.  This will
  	 * never cause a return when the relation is *not* locked, but will
  	 * occasionally let the check continue when there really *is* a relation
  	 * level lock.
  	 */
  	SET_PREDICATELOCKTARGETTAG_RELATION(tag,
  										relation->rd_node.dbNode,
  										relation->rd_id);
  	if (PredicateLockExists(&tag))
  		return;
  
  	tid = &(tuple->t_self);
  	SET_PREDICATELOCKTARGETTAG_TUPLE(tag,
  									 relation->rd_node.dbNode,
  									 relation->rd_id,
  									 ItemPointerGetBlockNumber(tid),
  									 ItemPointerGetOffsetNumber(tid));
  	PredicateLockAcquire(&tag);
  }
  
  
  /*
   *		DeleteLockTarget
   *
   * Remove a predicate lock target along with any locks held for it.
   *
   * Caller must hold SerializablePredicateLockListLock and the
   * appropriate hash partition lock for the target.
   */
  static void
  DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash)
  {
  	PREDICATELOCK *predlock;
  	SHM_QUEUE  *predlocktargetlink;
  	PREDICATELOCK *nextpredlock;
  	bool		found;
  
  	Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
  	Assert(LWLockHeldByMe(PredicateLockHashPartitionLock(targettaghash)));
  
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&(target->predicateLocks),
  					 &(target->predicateLocks),
  					 offsetof(PREDICATELOCK, targetLink));
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  	while (predlock)
  	{
  		predlocktargetlink = &(predlock->targetLink);
  		nextpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(target->predicateLocks),
  						 predlocktargetlink,
  						 offsetof(PREDICATELOCK, targetLink));
  
  		SHMQueueDelete(&(predlock->xactLink));
  		SHMQueueDelete(&(predlock->targetLink));
  
  		hash_search_with_hash_value
  			(PredicateLockHash,
  			 &predlock->tag,
  			 PredicateLockHashCodeFromTargetHashCode(&predlock->tag,
  													 targettaghash),
  			 HASH_REMOVE, &found);
  		Assert(found);
  
  		predlock = nextpredlock;
  	}
  	LWLockRelease(SerializableXactHashLock);
  
  	/* Remove the target itself, if possible. */
  	RemoveTargetIfNoLongerUsed(target, targettaghash);
  }
  
  
  /*
   *		TransferPredicateLocksToNewTarget
   *
   * Move or copy all the predicate locks for a lock target, for use by
   * index page splits/combines and other things that create or replace
   * lock targets. If 'removeOld' is true, the old locks and the target
   * will be removed.
   *
   * Returns true on success, or false if we ran out of shared memory to
   * allocate the new target or locks. Guaranteed to always succeed if
   * removeOld is set (by using the scratch entry in PredicateLockTargetHash
   * for scratch space).
   *
   * Warning: the "removeOld" option should be used only with care,
   * because this function does not (indeed, can not) update other
   * backends' LocalPredicateLockHash. If we are only adding new
   * entries, this is not a problem: the local lock table is used only
   * as a hint, so missing entries for locks that are held are
   * OK. Having entries for locks that are no longer held, as can happen
   * when using "removeOld", is not in general OK. We can only use it
   * safely when replacing a lock with a coarser-granularity lock that
   * covers it, or if we are absolutely certain that no one will need to
   * refer to that lock in the future.
   *
   * Caller must hold SerializablePredicateLockListLock.
   */
  static bool
  TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
  								  PREDICATELOCKTARGETTAG newtargettag,
  								  bool removeOld)
  {
  	uint32		oldtargettaghash;
  	LWLock	   *oldpartitionLock;
  	PREDICATELOCKTARGET *oldtarget;
  	uint32		newtargettaghash;
  	LWLock	   *newpartitionLock;
  	bool		found;
  	bool		outOfShmem = false;
  
  	Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
  
  	oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
  	newtargettaghash = PredicateLockTargetTagHashCode(&newtargettag);
  	oldpartitionLock = PredicateLockHashPartitionLock(oldtargettaghash);
  	newpartitionLock = PredicateLockHashPartitionLock(newtargettaghash);
  
  	if (removeOld)
  	{
  		/*
  		 * Remove the dummy entry to give us scratch space, so we know we'll
  		 * be able to create the new lock target.
  		 */
  		RemoveScratchTarget(false);
  	}
  
  	/*
  	 * We must get the partition locks in ascending sequence to avoid
  	 * deadlocks. If old and new partitions are the same, we must request the
  	 * lock only once.
  	 */
  	if (oldpartitionLock < newpartitionLock)
  	{
  		LWLockAcquire(oldpartitionLock,
  					  (removeOld ? LW_EXCLUSIVE : LW_SHARED));
  		LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
  	}
  	else if (oldpartitionLock > newpartitionLock)
  	{
  		LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
  		LWLockAcquire(oldpartitionLock,
  					  (removeOld ? LW_EXCLUSIVE : LW_SHARED));
  	}
  	else
  		LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
  
  	/*
  	 * Look for the old target.  If not found, that's OK; no predicate locks
  	 * are affected, so we can just clean up and return. If it does exist,
  	 * walk its list of predicate locks and move or copy them to the new
  	 * target.
  	 */
  	oldtarget = hash_search_with_hash_value(PredicateLockTargetHash,
  											&oldtargettag,
  											oldtargettaghash,
  											HASH_FIND, NULL);
  
  	if (oldtarget)
  	{
  		PREDICATELOCKTARGET *newtarget;
  		PREDICATELOCK *oldpredlock;
  		PREDICATELOCKTAG newpredlocktag;
  
  		newtarget = hash_search_with_hash_value(PredicateLockTargetHash,
  												&newtargettag,
  												newtargettaghash,
  												HASH_ENTER_NULL, &found);
  
  		if (!newtarget)
  		{
  			/* Failed to allocate due to insufficient shmem */
  			outOfShmem = true;
  			goto exit;
  		}
  
  		/* If we created a new entry, initialize it */
  		if (!found)
  			SHMQueueInit(&(newtarget->predicateLocks));
  
  		newpredlocktag.myTarget = newtarget;
  
  		/*
  		 * Loop through all the locks on the old target, replacing them with
  		 * locks on the new target.
  		 */
  		oldpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(oldtarget->predicateLocks),
  						 &(oldtarget->predicateLocks),
  						 offsetof(PREDICATELOCK, targetLink));
  		LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  		while (oldpredlock)
  		{
  			SHM_QUEUE  *predlocktargetlink;
  			PREDICATELOCK *nextpredlock;
  			PREDICATELOCK *newpredlock;
  			SerCommitSeqNo oldCommitSeqNo = oldpredlock->commitSeqNo;
  
  			predlocktargetlink = &(oldpredlock->targetLink);
  			nextpredlock = (PREDICATELOCK *)
  				SHMQueueNext(&(oldtarget->predicateLocks),
  							 predlocktargetlink,
  							 offsetof(PREDICATELOCK, targetLink));
  			newpredlocktag.myXact = oldpredlock->tag.myXact;
  
  			if (removeOld)
  			{
  				SHMQueueDelete(&(oldpredlock->xactLink));
  				SHMQueueDelete(&(oldpredlock->targetLink));
  
  				hash_search_with_hash_value
  					(PredicateLockHash,
  					 &oldpredlock->tag,
  					 PredicateLockHashCodeFromTargetHashCode(&oldpredlock->tag,
  															 oldtargettaghash),
  					 HASH_REMOVE, &found);
  				Assert(found);
  			}
  
  			newpredlock = (PREDICATELOCK *)
  				hash_search_with_hash_value(PredicateLockHash,
  											&newpredlocktag,
  											PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
  																					newtargettaghash),
  											HASH_ENTER_NULL,
  											&found);
  			if (!newpredlock)
  			{
  				/* Out of shared memory. Undo what we've done so far. */
  				LWLockRelease(SerializableXactHashLock);
  				DeleteLockTarget(newtarget, newtargettaghash);
  				outOfShmem = true;
  				goto exit;
  			}
  			if (!found)
  			{
  				SHMQueueInsertBefore(&(newtarget->predicateLocks),
  									 &(newpredlock->targetLink));
  				SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
  									 &(newpredlock->xactLink));
  				newpredlock->commitSeqNo = oldCommitSeqNo;
  			}
  			else
  			{
  				if (newpredlock->commitSeqNo < oldCommitSeqNo)
  					newpredlock->commitSeqNo = oldCommitSeqNo;
  			}
  
  			Assert(newpredlock->commitSeqNo != 0);
  			Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
  				   || (newpredlock->tag.myXact == OldCommittedSxact));
  
  			oldpredlock = nextpredlock;
  		}
  		LWLockRelease(SerializableXactHashLock);
  
  		if (removeOld)
  		{
  			Assert(SHMQueueEmpty(&oldtarget->predicateLocks));
  			RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);
  		}
  	}
  
  
  exit:
  	/* Release partition locks in reverse order of acquisition. */
  	if (oldpartitionLock < newpartitionLock)
  	{
  		LWLockRelease(newpartitionLock);
  		LWLockRelease(oldpartitionLock);
  	}
  	else if (oldpartitionLock > newpartitionLock)
  	{
  		LWLockRelease(oldpartitionLock);
  		LWLockRelease(newpartitionLock);
  	}
  	else
  		LWLockRelease(newpartitionLock);
  
  	if (removeOld)
  	{
  		/* We shouldn't run out of memory if we're moving locks */
  		Assert(!outOfShmem);
  
  		/* Put the scratch entry back */
  		RestoreScratchTarget(false);
  	}
  
  	return !outOfShmem;
  }
  
  /*
   * Drop all predicate locks of any granularity from the specified relation,
   * which can be a heap relation or an index relation.  If 'transfer' is true,
   * acquire a relation lock on the heap for any transactions with any lock(s)
   * on the specified relation.
   *
   * This requires grabbing a lot of LW locks and scanning the entire lock
   * target table for matches.  That makes this more expensive than most
   * predicate lock management functions, but it will only be called for DDL
   * type commands that are expensive anyway, and there are fast returns when
   * no serializable transactions are active or the relation is temporary.
   *
   * We don't use the TransferPredicateLocksToNewTarget function because it
   * acquires its own locks on the partitions of the two targets involved,
   * and we'll already be holding all partition locks.
   *
   * We can't throw an error from here, because the call could be from a
   * transaction which is not serializable.
   *
   * NOTE: This is currently only called with transfer set to true, but that may
   * change.  If we decide to clean up the locks from a table on commit of a
   * transaction which executed DROP TABLE, the false condition will be useful.
   */
  static void
  DropAllPredicateLocksFromTable(Relation relation, bool transfer)
  {
  	HASH_SEQ_STATUS seqstat;
  	PREDICATELOCKTARGET *oldtarget;
  	PREDICATELOCKTARGET *heaptarget;
  	Oid			dbId;
  	Oid			relId;
  	Oid			heapId;
  	int			i;
  	bool		isIndex;
  	bool		found;
  	uint32		heaptargettaghash;
  
  	/*
  	 * Bail out quickly if there are no serializable transactions running.
  	 * It's safe to check this without taking locks because the caller is
  	 * holding an ACCESS EXCLUSIVE lock on the relation.  No new locks which
  	 * would matter here can be acquired while that is held.
  	 */
  	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
  		return;
  
  	if (!PredicateLockingNeededForRelation(relation))
  		return;
  
  	dbId = relation->rd_node.dbNode;
  	relId = relation->rd_id;
  	if (relation->rd_index == NULL)
  	{
  		isIndex = false;
  		heapId = relId;
  	}
  	else
  	{
  		isIndex = true;
  		heapId = relation->rd_index->indrelid;
  	}
  	Assert(heapId != InvalidOid);
  	Assert(transfer || !isIndex);	/* index OID only makes sense with
  									 * transfer */
  
  	/* Retrieve first time needed, then keep. */
  	heaptargettaghash = 0;
  	heaptarget = NULL;
  
  	/* Acquire locks on all lock partitions */
  	LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);
  	for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
  		LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_EXCLUSIVE);
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	/*
  	 * Remove the dummy entry to give us scratch space, so we know we'll be
  	 * able to create the new lock target.
  	 */
  	if (transfer)
  		RemoveScratchTarget(true);
  
  	/* Scan through target map */
  	hash_seq_init(&seqstat, PredicateLockTargetHash);
  
  	while ((oldtarget = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
  	{
  		PREDICATELOCK *oldpredlock;
  
  		/*
  		 * Check whether this is a target which needs attention.
  		 */
  		if (GET_PREDICATELOCKTARGETTAG_RELATION(oldtarget->tag) != relId)
  			continue;			/* wrong relation id */
  		if (GET_PREDICATELOCKTARGETTAG_DB(oldtarget->tag) != dbId)
  			continue;			/* wrong database id */
  		if (transfer && !isIndex
  			&& GET_PREDICATELOCKTARGETTAG_TYPE(oldtarget->tag) == PREDLOCKTAG_RELATION)
  			continue;			/* already the right lock */
  
  		/*
  		 * If we made it here, we have work to do.  We make sure the heap
  		 * relation lock exists, then we walk the list of predicate locks for
  		 * the old target we found, moving all locks to the heap relation lock
  		 * -- unless they already hold that.
  		 */
  
  		/*
  		 * First make sure we have the heap relation target.  We only need to
  		 * do this once.
  		 */
  		if (transfer && heaptarget == NULL)
  		{
  			PREDICATELOCKTARGETTAG heaptargettag;
  
  			SET_PREDICATELOCKTARGETTAG_RELATION(heaptargettag, dbId, heapId);
  			heaptargettaghash = PredicateLockTargetTagHashCode(&heaptargettag);
  			heaptarget = hash_search_with_hash_value(PredicateLockTargetHash,
  													 &heaptargettag,
  													 heaptargettaghash,
  													 HASH_ENTER, &found);
  			if (!found)
  				SHMQueueInit(&heaptarget->predicateLocks);
  		}
  
  		/*
  		 * Loop through all the locks on the old target, replacing them with
  		 * locks on the new target.
  		 */
  		oldpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(oldtarget->predicateLocks),
  						 &(oldtarget->predicateLocks),
  						 offsetof(PREDICATELOCK, targetLink));
  		while (oldpredlock)
  		{
  			PREDICATELOCK *nextpredlock;
  			PREDICATELOCK *newpredlock;
  			SerCommitSeqNo oldCommitSeqNo;
  			SERIALIZABLEXACT *oldXact;
  
  			nextpredlock = (PREDICATELOCK *)
  				SHMQueueNext(&(oldtarget->predicateLocks),
  							 &(oldpredlock->targetLink),
  							 offsetof(PREDICATELOCK, targetLink));
  
  			/*
  			 * Remove the old lock first. This avoids the chance of running
  			 * out of lock structure entries for the hash table.
  			 */
  			oldCommitSeqNo = oldpredlock->commitSeqNo;
  			oldXact = oldpredlock->tag.myXact;
  
  			SHMQueueDelete(&(oldpredlock->xactLink));
  
  			/*
  			 * No need for retail delete from oldtarget list, we're removing
  			 * the whole target anyway.
  			 */
  			hash_search(PredicateLockHash,
  						&oldpredlock->tag,
  						HASH_REMOVE, &found);
  			Assert(found);
  
  			if (transfer)
  			{
  				PREDICATELOCKTAG newpredlocktag;
  
  				newpredlocktag.myTarget = heaptarget;
  				newpredlocktag.myXact = oldXact;
  				newpredlock = (PREDICATELOCK *)
  					hash_search_with_hash_value(PredicateLockHash,
  												&newpredlocktag,
  												PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
  																						heaptargettaghash),
  												HASH_ENTER,
  												&found);
  				if (!found)
  				{
  					SHMQueueInsertBefore(&(heaptarget->predicateLocks),
  										 &(newpredlock->targetLink));
  					SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
  										 &(newpredlock->xactLink));
  					newpredlock->commitSeqNo = oldCommitSeqNo;
  				}
  				else
  				{
  					if (newpredlock->commitSeqNo < oldCommitSeqNo)
  						newpredlock->commitSeqNo = oldCommitSeqNo;
  				}
  
  				Assert(newpredlock->commitSeqNo != 0);
  				Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
  					   || (newpredlock->tag.myXact == OldCommittedSxact));
  			}
  
  			oldpredlock = nextpredlock;
  		}
  
  		hash_search(PredicateLockTargetHash, &oldtarget->tag, HASH_REMOVE,
  					&found);
  		Assert(found);
  	}
  
  	/* Put the scratch entry back */
  	if (transfer)
  		RestoreScratchTarget(true);
  
  	/* Release locks in reverse order */
  	LWLockRelease(SerializableXactHashLock);
  	for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
  		LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  /*
   * TransferPredicateLocksToHeapRelation
   *		For all transactions, transfer all predicate locks for the given
   *		relation to a single relation lock on the heap.
   */
  void
  TransferPredicateLocksToHeapRelation(Relation relation)
  {
  	DropAllPredicateLocksFromTable(relation, true);
  }
  
  
  /*
   *		PredicateLockPageSplit
   *
   * Copies any predicate locks for the old page to the new page.
   * Skip if this is a temporary table or toast table.
   *
   * NOTE: A page split (or overflow) affects all serializable transactions,
   * even if it occurs in the context of another transaction isolation level.
   *
   * NOTE: This currently leaves the local copy of the locks without
   * information on the new lock which is in shared memory.  This could cause
   * problems if enough page splits occur on locked pages without the processes
   * which hold the locks getting in and noticing.
   */
  void
  PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
  					   BlockNumber newblkno)
  {
  	PREDICATELOCKTARGETTAG oldtargettag;
  	PREDICATELOCKTARGETTAG newtargettag;
  	bool		success;
  
  	/*
  	 * Bail out quickly if there are no serializable transactions running.
  	 *
  	 * It's safe to do this check without taking any additional locks. Even if
  	 * a serializable transaction starts concurrently, we know it can't take
  	 * any SIREAD locks on the page being split because the caller is holding
  	 * the associated buffer page lock. Memory reordering isn't an issue; the
  	 * memory barrier in the LWLock acquisition guarantees that this read
  	 * occurs while the buffer page lock is held.
  	 */
  	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
  		return;
  
  	if (!PredicateLockingNeededForRelation(relation))
  		return;
  
  	Assert(oldblkno != newblkno);
  	Assert(BlockNumberIsValid(oldblkno));
  	Assert(BlockNumberIsValid(newblkno));
  
  	SET_PREDICATELOCKTARGETTAG_PAGE(oldtargettag,
  									relation->rd_node.dbNode,
  									relation->rd_id,
  									oldblkno);
  	SET_PREDICATELOCKTARGETTAG_PAGE(newtargettag,
  									relation->rd_node.dbNode,
  									relation->rd_id,
  									newblkno);
  
  	LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);
  
  	/*
  	 * Try copying the locks over to the new page's tag, creating it if
  	 * necessary.
  	 */
  	success = TransferPredicateLocksToNewTarget(oldtargettag,
  												newtargettag,
  												false);
  
  	if (!success)
  	{
  		/*
  		 * No more predicate lock entries are available. Failure isn't an
  		 * option here, so promote the page lock to a relation lock.
  		 */
  
  		/* Get the parent relation lock's lock tag */
  		success = GetParentPredicateLockTag(&oldtargettag,
  											&newtargettag);
  		Assert(success);
  
  		/*
  		 * Move the locks to the parent. This shouldn't fail.
  		 *
  		 * Note that here we are removing locks held by other backends,
  		 * leading to a possible inconsistency in their local lock hash table.
  		 * This is OK because we're replacing it with a lock that covers the
  		 * old one.
  		 */
  		success = TransferPredicateLocksToNewTarget(oldtargettag,
  													newtargettag,
  													true);
  		Assert(success);
  	}
  
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  /*
   *		PredicateLockPageCombine
   *
   * Combines predicate locks for two existing pages.
   * Skip if this is a temporary table or toast table.
   *
   * NOTE: A page combine affects all serializable transactions, even if it
   * occurs in the context of another transaction isolation level.
   */
  void
  PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
  						 BlockNumber newblkno)
  {
  	/*
  	 * Page combines differ from page splits in that we ought to be able to
  	 * remove the locks on the old page after transferring them to the new
  	 * page, instead of duplicating them. However, because we can't edit other
  	 * backends' local lock tables, removing the old lock would leave them
  	 * with an entry in their LocalPredicateLockHash for a lock they're not
  	 * holding, which isn't acceptable. So we wind up having to do the same
  	 * work as a page split, acquiring a lock on the new page and keeping the
  	 * old page locked too. That can lead to some false positives, but should
  	 * be rare in practice.
  	 */
  	PredicateLockPageSplit(relation, oldblkno, newblkno);
  }
  
  /*
   * Walk the list of in-progress serializable transactions and find the new
   * xmin.
   */
  static void
  SetNewSxactGlobalXmin(void)
  {
  	SERIALIZABLEXACT *sxact;
  
  	Assert(LWLockHeldByMe(SerializableXactHashLock));
  
  	PredXact->SxactGlobalXmin = InvalidTransactionId;
  	PredXact->SxactGlobalXminCount = 0;
  
  	for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))
  	{
  		if (!SxactIsRolledBack(sxact)
  			&& !SxactIsCommitted(sxact)
  			&& sxact != OldCommittedSxact)
  		{
  			Assert(sxact->xmin != InvalidTransactionId);
  			if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
  				|| TransactionIdPrecedes(sxact->xmin,
  										 PredXact->SxactGlobalXmin))
  			{
  				PredXact->SxactGlobalXmin = sxact->xmin;
  				PredXact->SxactGlobalXminCount = 1;
  			}
  			else if (TransactionIdEquals(sxact->xmin,
  										 PredXact->SxactGlobalXmin))
  				PredXact->SxactGlobalXminCount++;
  		}
  	}
  
  	OldSerXidSetActiveSerXmin(PredXact->SxactGlobalXmin);
  }
  
  /*
   *		ReleasePredicateLocks
   *
   * Releases predicate locks based on completion of the current transaction,
   * whether committed or rolled back.  It can also be called for a read only
   * transaction when it becomes impossible for the transaction to become
   * part of a dangerous structure.
   *
   * We do nothing unless this is a serializable transaction.
   *
   * This method must ensure that shared memory hash tables are cleaned
   * up in some relatively timely fashion.
   *
   * If this transaction is committing and is holding any predicate locks,
   * it must be added to a list of completed serializable transactions still
   * holding locks.
   */
  void
  ReleasePredicateLocks(bool isCommit)
  {
  	bool		needToClear;
  	RWConflict	conflict,
  				nextConflict,
  				possibleUnsafeConflict;
  	SERIALIZABLEXACT *roXact;
  
  	/*
  	 * We can't trust XactReadOnly here, because a transaction which started
  	 * as READ WRITE can show as READ ONLY later, e.g., within
  	 * subtransactions.  We want to flag a transaction as READ ONLY if it
  	 * commits without writing so that de facto READ ONLY transactions get the
  	 * benefit of some RO optimizations, so we will use this local variable to
  	 * get some cleanup logic right which is based on whether the transaction
  	 * was declared READ ONLY at the top level.
  	 */
  	bool		topLevelIsDeclaredReadOnly;
  
  	if (MySerializableXact == InvalidSerializableXact)
  	{
  		Assert(LocalPredicateLockHash == NULL);
  		return;
  	}
  
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	Assert(!isCommit || SxactIsPrepared(MySerializableXact));
  	Assert(!isCommit || !SxactIsDoomed(MySerializableXact));
  	Assert(!SxactIsCommitted(MySerializableXact));
  	Assert(!SxactIsRolledBack(MySerializableXact));
  
  	/* may not be serializable during COMMIT/ROLLBACK PREPARED */
  	Assert(MySerializableXact->pid == 0 || IsolationIsSerializable());
  
  	/* We'd better not already be on the cleanup list. */
  	Assert(!SxactIsOnFinishedList(MySerializableXact));
  
  	topLevelIsDeclaredReadOnly = SxactIsReadOnly(MySerializableXact);
  
  	/*
  	 * We don't hold XidGenLock lock here, assuming that TransactionId is
  	 * atomic!
  	 *
  	 * If this value is changing, we don't care that much whether we get the
  	 * old or new value -- it is just used to determine how far
  	 * GlobalSerializableXmin must advance before this transaction can be
  	 * fully cleaned up.  The worst that could happen is we wait for one more
  	 * transaction to complete before freeing some RAM; correctness of visible
  	 * behavior is not affected.
  	 */
  	MySerializableXact->finishedBefore = ShmemVariableCache->nextXid;
  
  	/*
  	 * If it's not a commit it's a rollback, and we can clear our locks
  	 * immediately.
  	 */
  	if (isCommit)
  	{
  		MySerializableXact->flags |= SXACT_FLAG_COMMITTED;
  		MySerializableXact->commitSeqNo = ++(PredXact->LastSxactCommitSeqNo);
  		/* Recognize implicit read-only transaction (commit without write). */
  		if (!MyXactDidWrite)
  			MySerializableXact->flags |= SXACT_FLAG_READ_ONLY;
  	}
  	else
  	{
  		/*
  		 * The DOOMED flag indicates that we intend to roll back this
  		 * transaction and so it should not cause serialization failures for
  		 * other transactions that conflict with it. Note that this flag might
  		 * already be set, if another backend marked this transaction for
  		 * abort.
  		 *
  		 * The ROLLED_BACK flag further indicates that ReleasePredicateLocks
  		 * has been called, and so the SerializableXact is eligible for
  		 * cleanup. This means it should not be considered when calculating
  		 * SxactGlobalXmin.
  		 */
  		MySerializableXact->flags |= SXACT_FLAG_DOOMED;
  		MySerializableXact->flags |= SXACT_FLAG_ROLLED_BACK;
  
  		/*
  		 * If the transaction was previously prepared, but is now failing due
  		 * to a ROLLBACK PREPARED or (hopefully very rare) error after the
  		 * prepare, clear the prepared flag.  This simplifies conflict
  		 * checking.
  		 */
  		MySerializableXact->flags &= ~SXACT_FLAG_PREPARED;
  	}
  
  	if (!topLevelIsDeclaredReadOnly)
  	{
  		Assert(PredXact->WritableSxactCount > 0);
  		if (--(PredXact->WritableSxactCount) == 0)
  		{
  			/*
  			 * Release predicate locks and rw-conflicts in for all committed
  			 * transactions.  There are no longer any transactions which might
  			 * conflict with the locks and no chance for new transactions to
  			 * overlap.  Similarly, existing conflicts in can't cause pivots,
  			 * and any conflicts in which could have completed a dangerous
  			 * structure would already have caused a rollback, so any
  			 * remaining ones must be benign.
  			 */
  			PredXact->CanPartialClearThrough = PredXact->LastSxactCommitSeqNo;
  		}
  	}
  	else
  	{
  		/*
  		 * Read-only transactions: clear the list of transactions that might
  		 * make us unsafe. Note that we use 'inLink' for the iteration as
  		 * opposed to 'outLink' for the r/w xacts.
  		 */
  		possibleUnsafeConflict = (RWConflict)
  			SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
  						 &MySerializableXact->possibleUnsafeConflicts,
  						 offsetof(RWConflictData, inLink));
  		while (possibleUnsafeConflict)
  		{
  			nextConflict = (RWConflict)
  				SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
  							 &possibleUnsafeConflict->inLink,
  							 offsetof(RWConflictData, inLink));
  
  			Assert(!SxactIsReadOnly(possibleUnsafeConflict->sxactOut));
  			Assert(MySerializableXact == possibleUnsafeConflict->sxactIn);
  
  			ReleaseRWConflict(possibleUnsafeConflict);
  
  			possibleUnsafeConflict = nextConflict;
  		}
  	}
  
  	/* Check for conflict out to old committed transactions. */
  	if (isCommit
  		&& !SxactIsReadOnly(MySerializableXact)
  		&& SxactHasSummaryConflictOut(MySerializableXact))
  	{
  		/*
  		 * we don't know which old committed transaction we conflicted with,
  		 * so be conservative and use FirstNormalSerCommitSeqNo here
  		 */
  		MySerializableXact->SeqNo.earliestOutConflictCommit =
  			FirstNormalSerCommitSeqNo;
  		MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
  	}
  
  	/*
  	 * Release all outConflicts to committed transactions.  If we're rolling
  	 * back clear them all.  Set SXACT_FLAG_CONFLICT_OUT if any point to
  	 * previously committed transactions.
  	 */
  	conflict = (RWConflict)
  		SHMQueueNext(&MySerializableXact->outConflicts,
  					 &MySerializableXact->outConflicts,
  					 offsetof(RWConflictData, outLink));
  	while (conflict)
  	{
  		nextConflict = (RWConflict)
  			SHMQueueNext(&MySerializableXact->outConflicts,
  						 &conflict->outLink,
  						 offsetof(RWConflictData, outLink));
  
  		if (isCommit
  			&& !SxactIsReadOnly(MySerializableXact)
  			&& SxactIsCommitted(conflict->sxactIn))
  		{
  			if ((MySerializableXact->flags & SXACT_FLAG_CONFLICT_OUT) == 0
  				|| conflict->sxactIn->prepareSeqNo < MySerializableXact->SeqNo.earliestOutConflictCommit)
  				MySerializableXact->SeqNo.earliestOutConflictCommit = conflict->sxactIn->prepareSeqNo;
  			MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
  		}
  
  		if (!isCommit
  			|| SxactIsCommitted(conflict->sxactIn)
  			|| (conflict->sxactIn->SeqNo.lastCommitBeforeSnapshot >= PredXact->LastSxactCommitSeqNo))
  			ReleaseRWConflict(conflict);
  
  		conflict = nextConflict;
  	}
  
  	/*
  	 * Release all inConflicts from committed and read-only transactions. If
  	 * we're rolling back, clear them all.
  	 */
  	conflict = (RWConflict)
  		SHMQueueNext(&MySerializableXact->inConflicts,
  					 &MySerializableXact->inConflicts,
  					 offsetof(RWConflictData, inLink));
  	while (conflict)
  	{
  		nextConflict = (RWConflict)
  			SHMQueueNext(&MySerializableXact->inConflicts,
  						 &conflict->inLink,
  						 offsetof(RWConflictData, inLink));
  
  		if (!isCommit
  			|| SxactIsCommitted(conflict->sxactOut)
  			|| SxactIsReadOnly(conflict->sxactOut))
  			ReleaseRWConflict(conflict);
  
  		conflict = nextConflict;
  	}
  
  	if (!topLevelIsDeclaredReadOnly)
  	{
  		/*
  		 * Remove ourselves from the list of possible conflicts for concurrent
  		 * READ ONLY transactions, flagging them as unsafe if we have a
  		 * conflict out. If any are waiting DEFERRABLE transactions, wake them
  		 * up if they are known safe or known unsafe.
  		 */
  		possibleUnsafeConflict = (RWConflict)
  			SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
  						 &MySerializableXact->possibleUnsafeConflicts,
  						 offsetof(RWConflictData, outLink));
  		while (possibleUnsafeConflict)
  		{
  			nextConflict = (RWConflict)
  				SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
  							 &possibleUnsafeConflict->outLink,
  							 offsetof(RWConflictData, outLink));
  
  			roXact = possibleUnsafeConflict->sxactIn;
  			Assert(MySerializableXact == possibleUnsafeConflict->sxactOut);
  			Assert(SxactIsReadOnly(roXact));
  
  			/* Mark conflicted if necessary. */
  			if (isCommit
  				&& MyXactDidWrite
  				&& SxactHasConflictOut(MySerializableXact)
  				&& (MySerializableXact->SeqNo.earliestOutConflictCommit
  					<= roXact->SeqNo.lastCommitBeforeSnapshot))
  			{
  				/*
  				 * This releases possibleUnsafeConflict (as well as all other
  				 * possible conflicts for roXact)
  				 */
  				FlagSxactUnsafe(roXact);
  			}
  			else
  			{
  				ReleaseRWConflict(possibleUnsafeConflict);
  
  				/*
  				 * If we were the last possible conflict, flag it safe. The
  				 * transaction can now safely release its predicate locks (but
  				 * that transaction's backend has to do that itself).
  				 */
  				if (SHMQueueEmpty(&roXact->possibleUnsafeConflicts))
  					roXact->flags |= SXACT_FLAG_RO_SAFE;
  			}
  
  			/*
  			 * Wake up the process for a waiting DEFERRABLE transaction if we
  			 * now know it's either safe or conflicted.
  			 */
  			if (SxactIsDeferrableWaiting(roXact) &&
  				(SxactIsROUnsafe(roXact) || SxactIsROSafe(roXact)))
  				ProcSendSignal(roXact->pid);
  
  			possibleUnsafeConflict = nextConflict;
  		}
  	}
  
  	/*
  	 * Check whether it's time to clean up old transactions. This can only be
  	 * done when the last serializable transaction with the oldest xmin among
  	 * serializable transactions completes.  We then find the "new oldest"
  	 * xmin and purge any transactions which finished before this transaction
  	 * was launched.
  	 */
  	needToClear = false;
  	if (TransactionIdEquals(MySerializableXact->xmin, PredXact->SxactGlobalXmin))
  	{
  		Assert(PredXact->SxactGlobalXminCount > 0);
  		if (--(PredXact->SxactGlobalXminCount) == 0)
  		{
  			SetNewSxactGlobalXmin();
  			needToClear = true;
  		}
  	}
  
  	LWLockRelease(SerializableXactHashLock);
  
  	LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
  
  	/* Add this to the list of transactions to check for later cleanup. */
  	if (isCommit)
  		SHMQueueInsertBefore(FinishedSerializableTransactions,
  							 &MySerializableXact->finishedLink);
  
  	if (!isCommit)
  		ReleaseOneSerializableXact(MySerializableXact, false, false);
  
  	LWLockRelease(SerializableFinishedListLock);
  
  	if (needToClear)
  		ClearOldPredicateLocks();
  
  	MySerializableXact = InvalidSerializableXact;
  	MyXactDidWrite = false;
  
  	/* Delete per-transaction lock table */
  	if (LocalPredicateLockHash != NULL)
  	{
  		hash_destroy(LocalPredicateLockHash);
  		LocalPredicateLockHash = NULL;
  	}
  }
  
  /*
   * Clear old predicate locks, belonging to committed transactions that are no
   * longer interesting to any in-progress transaction.
   */
  static void
  ClearOldPredicateLocks(void)
  {
  	SERIALIZABLEXACT *finishedSxact;
  	PREDICATELOCK *predlock;
  
  	/*
  	 * Loop through finished transactions. They are in commit order, so we can
  	 * stop as soon as we find one that's still interesting.
  	 */
  	LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
  	finishedSxact = (SERIALIZABLEXACT *)
  		SHMQueueNext(FinishedSerializableTransactions,
  					 FinishedSerializableTransactions,
  					 offsetof(SERIALIZABLEXACT, finishedLink));
  	LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  	while (finishedSxact)
  	{
  		SERIALIZABLEXACT *nextSxact;
  
  		nextSxact = (SERIALIZABLEXACT *)
  			SHMQueueNext(FinishedSerializableTransactions,
  						 &(finishedSxact->finishedLink),
  						 offsetof(SERIALIZABLEXACT, finishedLink));
  		if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
  			|| TransactionIdPrecedesOrEquals(finishedSxact->finishedBefore,
  											 PredXact->SxactGlobalXmin))
  		{
  			/*
  			 * This transaction committed before any in-progress transaction
  			 * took its snapshot. It's no longer interesting.
  			 */
  			LWLockRelease(SerializableXactHashLock);
  			SHMQueueDelete(&(finishedSxact->finishedLink));
  			ReleaseOneSerializableXact(finishedSxact, false, false);
  			LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  		}
  		else if (finishedSxact->commitSeqNo > PredXact->HavePartialClearedThrough
  				 && finishedSxact->commitSeqNo <= PredXact->CanPartialClearThrough)
  		{
  			/*
  			 * Any active transactions that took their snapshot before this
  			 * transaction committed are read-only, so we can clear part of
  			 * its state.
  			 */
  			LWLockRelease(SerializableXactHashLock);
  
  			if (SxactIsReadOnly(finishedSxact))
  			{
  				/* A read-only transaction can be removed entirely */
  				SHMQueueDelete(&(finishedSxact->finishedLink));
  				ReleaseOneSerializableXact(finishedSxact, false, false);
  			}
  			else
  			{
  				/*
  				 * A read-write transaction can only be partially cleared. We
  				 * need to keep the SERIALIZABLEXACT but can release the
  				 * SIREAD locks and conflicts in.
  				 */
  				ReleaseOneSerializableXact(finishedSxact, true, false);
  			}
  
  			PredXact->HavePartialClearedThrough = finishedSxact->commitSeqNo;
  			LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  		}
  		else
  		{
  			/* Still interesting. */
  			break;
  		}
  		finishedSxact = nextSxact;
  	}
  	LWLockRelease(SerializableXactHashLock);
  
  	/*
  	 * Loop through predicate locks on dummy transaction for summarized data.
  	 */
  	LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&OldCommittedSxact->predicateLocks,
  					 &OldCommittedSxact->predicateLocks,
  					 offsetof(PREDICATELOCK, xactLink));
  	while (predlock)
  	{
  		PREDICATELOCK *nextpredlock;
  		bool		canDoPartialCleanup;
  
  		nextpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&OldCommittedSxact->predicateLocks,
  						 &predlock->xactLink,
  						 offsetof(PREDICATELOCK, xactLink));
  
  		LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  		Assert(predlock->commitSeqNo != 0);
  		Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
  		canDoPartialCleanup = (predlock->commitSeqNo <= PredXact->CanPartialClearThrough);
  		LWLockRelease(SerializableXactHashLock);
  
  		/*
  		 * If this lock originally belonged to an old enough transaction, we
  		 * can release it.
  		 */
  		if (canDoPartialCleanup)
  		{
  			PREDICATELOCKTAG tag;
  			PREDICATELOCKTARGET *target;
  			PREDICATELOCKTARGETTAG targettag;
  			uint32		targettaghash;
  			LWLock	   *partitionLock;
  
  			tag = predlock->tag;
  			target = tag.myTarget;
  			targettag = target->tag;
  			targettaghash = PredicateLockTargetTagHashCode(&targettag);
  			partitionLock = PredicateLockHashPartitionLock(targettaghash);
  
  			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
  
  			SHMQueueDelete(&(predlock->targetLink));
  			SHMQueueDelete(&(predlock->xactLink));
  
  			hash_search_with_hash_value(PredicateLockHash, &tag,
  										PredicateLockHashCodeFromTargetHashCode(&tag,
  																				targettaghash),
  										HASH_REMOVE, NULL);
  			RemoveTargetIfNoLongerUsed(target, targettaghash);
  
  			LWLockRelease(partitionLock);
  		}
  
  		predlock = nextpredlock;
  	}
  
  	LWLockRelease(SerializablePredicateLockListLock);
  	LWLockRelease(SerializableFinishedListLock);
  }
  
  /*
   * This is the normal way to delete anything from any of the predicate
   * locking hash tables.  Given a transaction which we know can be deleted:
   * delete all predicate locks held by that transaction and any predicate
   * lock targets which are now unreferenced by a lock; delete all conflicts
   * for the transaction; delete all xid values for the transaction; then
   * delete the transaction.
   *
   * When the partial flag is set, we can release all predicate locks and
   * in-conflict information -- we've established that there are no longer
   * any overlapping read write transactions for which this transaction could
   * matter -- but keep the transaction entry itself and any outConflicts.
   *
   * When the summarize flag is set, we've run short of room for sxact data
   * and must summarize to the SLRU.  Predicate locks are transferred to a
   * dummy "old" transaction, with duplicate locks on a single target
   * collapsing to a single lock with the "latest" commitSeqNo from among
   * the conflicting locks..
   */
  static void
  ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
  						   bool summarize)
  {
  	PREDICATELOCK *predlock;
  	SERIALIZABLEXIDTAG sxidtag;
  	RWConflict	conflict,
  				nextConflict;
  
  	Assert(sxact != NULL);
  	Assert(SxactIsRolledBack(sxact) || SxactIsCommitted(sxact));
  	Assert(partial || !SxactIsOnFinishedList(sxact));
  	Assert(LWLockHeldByMe(SerializableFinishedListLock));
  
  	/*
  	 * First release all the predicate locks held by this xact (or transfer
  	 * them to OldCommittedSxact if summarize is true)
  	 */
  	LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&(sxact->predicateLocks),
  					 &(sxact->predicateLocks),
  					 offsetof(PREDICATELOCK, xactLink));
  	while (predlock)
  	{
  		PREDICATELOCK *nextpredlock;
  		PREDICATELOCKTAG tag;
  		SHM_QUEUE  *targetLink;
  		PREDICATELOCKTARGET *target;
  		PREDICATELOCKTARGETTAG targettag;
  		uint32		targettaghash;
  		LWLock	   *partitionLock;
  
  		nextpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(sxact->predicateLocks),
  						 &(predlock->xactLink),
  						 offsetof(PREDICATELOCK, xactLink));
  
  		tag = predlock->tag;
  		targetLink = &(predlock->targetLink);
  		target = tag.myTarget;
  		targettag = target->tag;
  		targettaghash = PredicateLockTargetTagHashCode(&targettag);
  		partitionLock = PredicateLockHashPartitionLock(targettaghash);
  
  		LWLockAcquire(partitionLock, LW_EXCLUSIVE);
  
  		SHMQueueDelete(targetLink);
  
  		hash_search_with_hash_value(PredicateLockHash, &tag,
  									PredicateLockHashCodeFromTargetHashCode(&tag,
  																			targettaghash),
  									HASH_REMOVE, NULL);
  		if (summarize)
  		{
  			bool		found;
  
  			/* Fold into dummy transaction list. */
  			tag.myXact = OldCommittedSxact;
  			predlock = hash_search_with_hash_value(PredicateLockHash, &tag,
  												   PredicateLockHashCodeFromTargetHashCode(&tag,
  																						   targettaghash),
  												   HASH_ENTER_NULL, &found);
  			if (!predlock)
  				ereport(ERROR,
  						(errcode(ERRCODE_OUT_OF_MEMORY),
  						 errmsg("out of shared memory"),
  						 errhint("You might need to increase max_pred_locks_per_transaction.")));
  			if (found)
  			{
  				Assert(predlock->commitSeqNo != 0);
  				Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
  				if (predlock->commitSeqNo < sxact->commitSeqNo)
  					predlock->commitSeqNo = sxact->commitSeqNo;
  			}
  			else
  			{
  				SHMQueueInsertBefore(&(target->predicateLocks),
  									 &(predlock->targetLink));
  				SHMQueueInsertBefore(&(OldCommittedSxact->predicateLocks),
  									 &(predlock->xactLink));
  				predlock->commitSeqNo = sxact->commitSeqNo;
  			}
  		}
  		else
  			RemoveTargetIfNoLongerUsed(target, targettaghash);
  
  		LWLockRelease(partitionLock);
  
  		predlock = nextpredlock;
  	}
  
  	/*
  	 * Rather than retail removal, just re-init the head after we've run
  	 * through the list.
  	 */
  	SHMQueueInit(&sxact->predicateLocks);
  
  	LWLockRelease(SerializablePredicateLockListLock);
  
  	sxidtag.xid = sxact->topXid;
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	/* Release all outConflicts (unless 'partial' is true) */
  	if (!partial)
  	{
  		conflict = (RWConflict)
  			SHMQueueNext(&sxact->outConflicts,
  						 &sxact->outConflicts,
  						 offsetof(RWConflictData, outLink));
  		while (conflict)
  		{
  			nextConflict = (RWConflict)
  				SHMQueueNext(&sxact->outConflicts,
  							 &conflict->outLink,
  							 offsetof(RWConflictData, outLink));
  			if (summarize)
  				conflict->sxactIn->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
  			ReleaseRWConflict(conflict);
  			conflict = nextConflict;
  		}
  	}
  
  	/* Release all inConflicts. */
  	conflict = (RWConflict)
  		SHMQueueNext(&sxact->inConflicts,
  					 &sxact->inConflicts,
  					 offsetof(RWConflictData, inLink));
  	while (conflict)
  	{
  		nextConflict = (RWConflict)
  			SHMQueueNext(&sxact->inConflicts,
  						 &conflict->inLink,
  						 offsetof(RWConflictData, inLink));
  		if (summarize)
  			conflict->sxactOut->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
  		ReleaseRWConflict(conflict);
  		conflict = nextConflict;
  	}
  
  	/* Finally, get rid of the xid and the record of the transaction itself. */
  	if (!partial)
  	{
  		if (sxidtag.xid != InvalidTransactionId)
  			hash_search(SerializableXidHash, &sxidtag, HASH_REMOVE, NULL);
  		ReleasePredXact(sxact);
  	}
  
  	LWLockRelease(SerializableXactHashLock);
  }
  
  /*
   * Tests whether the given top level transaction is concurrent with
   * (overlaps) our current transaction.
   *
   * We need to identify the top level transaction for SSI, anyway, so pass
   * that to this function to save the overhead of checking the snapshot's
   * subxip array.
   */
  static bool
  XidIsConcurrent(TransactionId xid)
  {
  	Snapshot	snap;
  	uint32		i;
  
  	Assert(TransactionIdIsValid(xid));
  	Assert(!TransactionIdEquals(xid, GetTopTransactionIdIfAny()));
  
  	snap = GetTransactionSnapshot();
  
  	if (TransactionIdPrecedes(xid, snap->xmin))
  		return false;
  
  	if (TransactionIdFollowsOrEquals(xid, snap->xmax))
  		return true;
  
  	for (i = 0; i < snap->xcnt; i++)
  	{
  		if (xid == snap->xip[i])
  			return true;
  	}
  
  	return false;
  }
  
  /*
   * CheckForSerializableConflictOut
   *		We are reading a tuple which has been modified.  If it is visible to
   *		us but has been deleted, that indicates a rw-conflict out.  If it's
   *		not visible and was created by a concurrent (overlapping)
   *		serializable transaction, that is also a rw-conflict out,
   *
   * We will determine the top level xid of the writing transaction with which
   * we may be in conflict, and check for overlap with our own transaction.
   * If the transactions overlap (i.e., they cannot see each other's writes),
   * then we have a conflict out.
   *
   * This function should be called just about anywhere in heapam.c where a
   * tuple has been read. The caller must hold at least a shared lock on the
   * buffer, because this function might set hint bits on the tuple. There is
   * currently no known reason to call this function from an index AM.
   */
  void
  CheckForSerializableConflictOut(bool visible, Relation relation,
  								HeapTuple tuple, Buffer buffer,
  								Snapshot snapshot)
  {
  	TransactionId xid;
  	SERIALIZABLEXIDTAG sxidtag;
  	SERIALIZABLEXID *sxid;
  	SERIALIZABLEXACT *sxact;
  	HTSV_Result htsvResult;
  
  	if (!SerializationNeededForRead(relation, snapshot))
  		return;
  
  	/* Check if someone else has already decided that we need to die */
  	if (SxactIsDoomed(MySerializableXact))
  	{
  		ereport(ERROR,
  				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  				 errmsg("could not serialize access due to read/write dependencies among transactions"),
  				 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict out checking."),
  				 errhint("The transaction might succeed if retried.")));
  	}
  
  	/*
  	 * Check to see whether the tuple has been written to by a concurrent
  	 * transaction, either to create it not visible to us, or to delete it
  	 * while it is visible to us.  The "visible" bool indicates whether the
  	 * tuple is visible to us, while HeapTupleSatisfiesVacuum checks what else
  	 * is going on with it.
  	 */
  	htsvResult = HeapTupleSatisfiesVacuum(tuple, TransactionXmin, buffer);
  	switch (htsvResult)
  	{
  		case HEAPTUPLE_LIVE:
  			if (visible)
  				return;
  			xid = HeapTupleHeaderGetXmin(tuple->t_data);
  			break;
  		case HEAPTUPLE_RECENTLY_DEAD:
  			if (!visible)
  				return;
  			xid = HeapTupleHeaderGetUpdateXid(tuple->t_data);
  			break;
  		case HEAPTUPLE_DELETE_IN_PROGRESS:
  			xid = HeapTupleHeaderGetUpdateXid(tuple->t_data);
  			break;
  		case HEAPTUPLE_INSERT_IN_PROGRESS:
  			xid = HeapTupleHeaderGetXmin(tuple->t_data);
  			break;
  		case HEAPTUPLE_DEAD:
  			return;
  		default:
  
  			/*
  			 * The only way to get to this default clause is if a new value is
  			 * added to the enum type without adding it to this switch
  			 * statement.  That's a bug, so elog.
  			 */
  			elog(ERROR, "unrecognized return value from HeapTupleSatisfiesVacuum: %u", htsvResult);
  
  			/*
  			 * In spite of having all enum values covered and calling elog on
  			 * this default, some compilers think this is a code path which
  			 * allows xid to be used below without initialization. Silence
  			 * that warning.
  			 */
  			xid = InvalidTransactionId;
  	}
  	Assert(TransactionIdIsValid(xid));
  	Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));
  
  	/*
  	 * Find top level xid.  Bail out if xid is too early to be a conflict, or
  	 * if it's our own xid.
  	 */
  	if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
  		return;
  	xid = SubTransGetTopmostTransaction(xid);
  	if (TransactionIdPrecedes(xid, TransactionXmin))
  		return;
  	if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
  		return;
  
  	/*
  	 * Find sxact or summarized info for the top level xid.
  	 */
  	sxidtag.xid = xid;
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  	sxid = (SERIALIZABLEXID *)
  		hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
  	if (!sxid)
  	{
  		/*
  		 * Transaction not found in "normal" SSI structures.  Check whether it
  		 * got pushed out to SLRU storage for "old committed" transactions.
  		 */
  		SerCommitSeqNo conflictCommitSeqNo;
  
  		conflictCommitSeqNo = OldSerXidGetMinConflictCommitSeqNo(xid);
  		if (conflictCommitSeqNo != 0)
  		{
  			if (conflictCommitSeqNo != InvalidSerCommitSeqNo
  				&& (!SxactIsReadOnly(MySerializableXact)
  					|| conflictCommitSeqNo
  					<= MySerializableXact->SeqNo.lastCommitBeforeSnapshot))
  				ereport(ERROR,
  						(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  						 errmsg("could not serialize access due to read/write dependencies among transactions"),
  						 errdetail_internal("Reason code: Canceled on conflict out to old pivot %u.", xid),
  						 errhint("The transaction might succeed if retried.")));
  
  			if (SxactHasSummaryConflictIn(MySerializableXact)
  				|| !SHMQueueEmpty(&MySerializableXact->inConflicts))
  				ereport(ERROR,
  						(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  						 errmsg("could not serialize access due to read/write dependencies among transactions"),
  						 errdetail_internal("Reason code: Canceled on identification as a pivot, with conflict out to old committed transaction %u.", xid),
  						 errhint("The transaction might succeed if retried.")));
  
  			MySerializableXact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
  		}
  
  		/* It's not serializable or otherwise not important. */
  		LWLockRelease(SerializableXactHashLock);
  		return;
  	}
  	sxact = sxid->myXact;
  	Assert(TransactionIdEquals(sxact->topXid, xid));
  	if (sxact == MySerializableXact || SxactIsDoomed(sxact))
  	{
  		/* Can't conflict with ourself or a transaction that will roll back. */
  		LWLockRelease(SerializableXactHashLock);
  		return;
  	}
  
  	/*
  	 * We have a conflict out to a transaction which has a conflict out to a
  	 * summarized transaction.  That summarized transaction must have
  	 * committed first, and we can't tell when it committed in relation to our
  	 * snapshot acquisition, so something needs to be canceled.
  	 */
  	if (SxactHasSummaryConflictOut(sxact))
  	{
  		if (!SxactIsPrepared(sxact))
  		{
  			sxact->flags |= SXACT_FLAG_DOOMED;
  			LWLockRelease(SerializableXactHashLock);
  			return;
  		}
  		else
  		{
  			LWLockRelease(SerializableXactHashLock);
  			ereport(ERROR,
  					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  					 errmsg("could not serialize access due to read/write dependencies among transactions"),
  					 errdetail_internal("Reason code: Canceled on conflict out to old pivot."),
  					 errhint("The transaction might succeed if retried.")));
  		}
  	}
  
  	/*
  	 * If this is a read-only transaction and the writing transaction has
  	 * committed, and it doesn't have a rw-conflict to a transaction which
  	 * committed before it, no conflict.
  	 */
  	if (SxactIsReadOnly(MySerializableXact)
  		&& SxactIsCommitted(sxact)
  		&& !SxactHasSummaryConflictOut(sxact)
  		&& (!SxactHasConflictOut(sxact)
  			|| MySerializableXact->SeqNo.lastCommitBeforeSnapshot < sxact->SeqNo.earliestOutConflictCommit))
  	{
  		/* Read-only transaction will appear to run first.  No conflict. */
  		LWLockRelease(SerializableXactHashLock);
  		return;
  	}
  
  	if (!XidIsConcurrent(xid))
  	{
  		/* This write was already in our snapshot; no conflict. */
  		LWLockRelease(SerializableXactHashLock);
  		return;
  	}
  
  	if (RWConflictExists(MySerializableXact, sxact))
  	{
  		/* We don't want duplicate conflict records in the list. */
  		LWLockRelease(SerializableXactHashLock);
  		return;
  	}
  
  	/*
  	 * Flag the conflict.  But first, if this conflict creates a dangerous
  	 * structure, ereport an error.
  	 */
  	FlagRWConflict(MySerializableXact, sxact);
  	LWLockRelease(SerializableXactHashLock);
  }
  
  /*
   * Check a particular target for rw-dependency conflict in. A subroutine of
   * CheckForSerializableConflictIn().
   */
  static void
  CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
  {
  	uint32		targettaghash;
  	LWLock	   *partitionLock;
  	PREDICATELOCKTARGET *target;
  	PREDICATELOCK *predlock;
  	PREDICATELOCK *mypredlock = NULL;
  	PREDICATELOCKTAG mypredlocktag;
  
  	Assert(MySerializableXact != InvalidSerializableXact);
  
  	/*
  	 * The same hash and LW lock apply to the lock target and the lock itself.
  	 */
  	targettaghash = PredicateLockTargetTagHashCode(targettag);
  	partitionLock = PredicateLockHashPartitionLock(targettaghash);
  	LWLockAcquire(partitionLock, LW_SHARED);
  	target = (PREDICATELOCKTARGET *)
  		hash_search_with_hash_value(PredicateLockTargetHash,
  									targettag, targettaghash,
  									HASH_FIND, NULL);
  	if (!target)
  	{
  		/* Nothing has this target locked; we're done here. */
  		LWLockRelease(partitionLock);
  		return;
  	}
  
  	/*
  	 * Each lock for an overlapping transaction represents a conflict: a
  	 * rw-dependency in to this transaction.
  	 */
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&(target->predicateLocks),
  					 &(target->predicateLocks),
  					 offsetof(PREDICATELOCK, targetLink));
  	LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  	while (predlock)
  	{
  		SHM_QUEUE  *predlocktargetlink;
  		PREDICATELOCK *nextpredlock;
  		SERIALIZABLEXACT *sxact;
  
  		predlocktargetlink = &(predlock->targetLink);
  		nextpredlock = (PREDICATELOCK *)
  			SHMQueueNext(&(target->predicateLocks),
  						 predlocktargetlink,
  						 offsetof(PREDICATELOCK, targetLink));
  
  		sxact = predlock->tag.myXact;
  		if (sxact == MySerializableXact)
  		{
  			/*
  			 * If we're getting a write lock on a tuple, we don't need a
  			 * predicate (SIREAD) lock on the same tuple. We can safely remove
  			 * our SIREAD lock, but we'll defer doing so until after the loop
  			 * because that requires upgrading to an exclusive partition lock.
  			 *
  			 * We can't use this optimization within a subtransaction because
  			 * the subtransaction could roll back, and we would be left
  			 * without any lock at the top level.
  			 */
  			if (!IsSubTransaction()
  				&& GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
  			{
  				mypredlock = predlock;
  				mypredlocktag = predlock->tag;
  			}
  		}
  		else if (!SxactIsDoomed(sxact)
  				 && (!SxactIsCommitted(sxact)
  					 || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
  											  sxact->finishedBefore))
  				 && !RWConflictExists(sxact, MySerializableXact))
  		{
  			LWLockRelease(SerializableXactHashLock);
  			LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  			/*
  			 * Re-check after getting exclusive lock because the other
  			 * transaction may have flagged a conflict.
  			 */
  			if (!SxactIsDoomed(sxact)
  				&& (!SxactIsCommitted(sxact)
  					|| TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
  											 sxact->finishedBefore))
  				&& !RWConflictExists(sxact, MySerializableXact))
  			{
  				FlagRWConflict(sxact, MySerializableXact);
  			}
  
  			LWLockRelease(SerializableXactHashLock);
  			LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  		}
  
  		predlock = nextpredlock;
  	}
  	LWLockRelease(SerializableXactHashLock);
  	LWLockRelease(partitionLock);
  
  	/*
  	 * If we found one of our own SIREAD locks to remove, remove it now.
  	 *
  	 * At this point our transaction already has an ExclusiveRowLock on the
  	 * relation, so we are OK to drop the predicate lock on the tuple, if
  	 * found, without fearing that another write against the tuple will occur
  	 * before the MVCC information makes it to the buffer.
  	 */
  	if (mypredlock != NULL)
  	{
  		uint32		predlockhashcode;
  		PREDICATELOCK *rmpredlock;
  
  		LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  		LWLockAcquire(partitionLock, LW_EXCLUSIVE);
  		LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  		/*
  		 * Remove the predicate lock from shared memory, if it wasn't removed
  		 * while the locks were released.  One way that could happen is from
  		 * autovacuum cleaning up an index.
  		 */
  		predlockhashcode = PredicateLockHashCodeFromTargetHashCode
  			(&mypredlocktag, targettaghash);
  		rmpredlock = (PREDICATELOCK *)
  			hash_search_with_hash_value(PredicateLockHash,
  										&mypredlocktag,
  										predlockhashcode,
  										HASH_FIND, NULL);
  		if (rmpredlock != NULL)
  		{
  			Assert(rmpredlock == mypredlock);
  
  			SHMQueueDelete(&(mypredlock->targetLink));
  			SHMQueueDelete(&(mypredlock->xactLink));
  
  			rmpredlock = (PREDICATELOCK *)
  				hash_search_with_hash_value(PredicateLockHash,
  											&mypredlocktag,
  											predlockhashcode,
  											HASH_REMOVE, NULL);
  			Assert(rmpredlock == mypredlock);
  
  			RemoveTargetIfNoLongerUsed(target, targettaghash);
  		}
  
  		LWLockRelease(SerializableXactHashLock);
  		LWLockRelease(partitionLock);
  		LWLockRelease(SerializablePredicateLockListLock);
  
  		if (rmpredlock != NULL)
  		{
  			/*
  			 * Remove entry in local lock table if it exists. It's OK if it
  			 * doesn't exist; that means the lock was transferred to a new
  			 * target by a different backend.
  			 */
  			hash_search_with_hash_value(LocalPredicateLockHash,
  										targettag, targettaghash,
  										HASH_REMOVE, NULL);
  
  			DecrementParentLocks(targettag);
  		}
  	}
  }
  
  /*
   * CheckForSerializableConflictIn
   *		We are writing the given tuple.  If that indicates a rw-conflict
   *		in from another serializable transaction, take appropriate action.
   *
   * Skip checking for any granularity for which a parameter is missing.
   *
   * A tuple update or delete is in conflict if we have a predicate lock
   * against the relation or page in which the tuple exists, or against the
   * tuple itself.
   */
  void
  CheckForSerializableConflictIn(Relation relation, HeapTuple tuple,
  							   Buffer buffer)
  {
  	PREDICATELOCKTARGETTAG targettag;
  
  	if (!SerializationNeededForWrite(relation))
  		return;
  
  	/* Check if someone else has already decided that we need to die */
  	if (SxactIsDoomed(MySerializableXact))
  		ereport(ERROR,
  				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  				 errmsg("could not serialize access due to read/write dependencies among transactions"),
  				 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict in checking."),
  				 errhint("The transaction might succeed if retried.")));
  
  	/*
  	 * We're doing a write which might cause rw-conflicts now or later.
  	 * Memorize that fact.
  	 */
  	MyXactDidWrite = true;
  
  	/*
  	 * It is important that we check for locks from the finest granularity to
  	 * the coarsest granularity, so that granularity promotion doesn't cause
  	 * us to miss a lock.  The new (coarser) lock will be acquired before the
  	 * old (finer) locks are released.
  	 *
  	 * It is not possible to take and hold a lock across the checks for all
  	 * granularities because each target could be in a separate partition.
  	 */
  	if (tuple != NULL)
  	{
  		SET_PREDICATELOCKTARGETTAG_TUPLE(targettag,
  										 relation->rd_node.dbNode,
  										 relation->rd_id,
  										 ItemPointerGetBlockNumber(&(tuple->t_self)),
  										 ItemPointerGetOffsetNumber(&(tuple->t_self)));
  		CheckTargetForConflictsIn(&targettag);
  	}
  
  	if (BufferIsValid(buffer))
  	{
  		SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
  										relation->rd_node.dbNode,
  										relation->rd_id,
  										BufferGetBlockNumber(buffer));
  		CheckTargetForConflictsIn(&targettag);
  	}
  
  	SET_PREDICATELOCKTARGETTAG_RELATION(targettag,
  										relation->rd_node.dbNode,
  										relation->rd_id);
  	CheckTargetForConflictsIn(&targettag);
  }
  
  /*
   * CheckTableForSerializableConflictIn
   *		The entire table is going through a DDL-style logical mass delete
   *		like TRUNCATE or DROP TABLE.  If that causes a rw-conflict in from
   *		another serializable transaction, take appropriate action.
   *
   * While these operations do not operate entirely within the bounds of
   * snapshot isolation, they can occur inside a serializable transaction, and
   * will logically occur after any reads which saw rows which were destroyed
   * by these operations, so we do what we can to serialize properly under
   * SSI.
   *
   * The relation passed in must be a heap relation. Any predicate lock of any
   * granularity on the heap will cause a rw-conflict in to this transaction.
   * Predicate locks on indexes do not matter because they only exist to guard
   * against conflicting inserts into the index, and this is a mass *delete*.
   * When a table is truncated or dropped, the index will also be truncated
   * or dropped, and we'll deal with locks on the index when that happens.
   *
   * Dropping or truncating a table also needs to drop any existing predicate
   * locks on heap tuples or pages, because they're about to go away. This
   * should be done before altering the predicate locks because the transaction
   * could be rolled back because of a conflict, in which case the lock changes
   * are not needed. (At the moment, we don't actually bother to drop the
   * existing locks on a dropped or truncated table at the moment. That might
   * lead to some false positives, but it doesn't seem worth the trouble.)
   */
  void
  CheckTableForSerializableConflictIn(Relation relation)
  {
  	HASH_SEQ_STATUS seqstat;
  	PREDICATELOCKTARGET *target;
  	Oid			dbId;
  	Oid			heapId;
  	int			i;
  
  	/*
  	 * Bail out quickly if there are no serializable transactions running.
  	 * It's safe to check this without taking locks because the caller is
  	 * holding an ACCESS EXCLUSIVE lock on the relation.  No new locks which
  	 * would matter here can be acquired while that is held.
  	 */
  	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
  		return;
  
  	if (!SerializationNeededForWrite(relation))
  		return;
  
  	/*
  	 * We're doing a write which might cause rw-conflicts now or later.
  	 * Memorize that fact.
  	 */
  	MyXactDidWrite = true;
  
  	Assert(relation->rd_index == NULL); /* not an index relation */
  
  	dbId = relation->rd_node.dbNode;
  	heapId = relation->rd_id;
  
  	LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);
  	for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
  		LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_SHARED);
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	/* Scan through target list */
  	hash_seq_init(&seqstat, PredicateLockTargetHash);
  
  	while ((target = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
  	{
  		PREDICATELOCK *predlock;
  
  		/*
  		 * Check whether this is a target which needs attention.
  		 */
  		if (GET_PREDICATELOCKTARGETTAG_RELATION(target->tag) != heapId)
  			continue;			/* wrong relation id */
  		if (GET_PREDICATELOCKTARGETTAG_DB(target->tag) != dbId)
  			continue;			/* wrong database id */
  
  		/*
  		 * Loop through locks for this target and flag conflicts.
  		 */
  		predlock = (PREDICATELOCK *)
  			SHMQueueNext(&(target->predicateLocks),
  						 &(target->predicateLocks),
  						 offsetof(PREDICATELOCK, targetLink));
  		while (predlock)
  		{
  			PREDICATELOCK *nextpredlock;
  
  			nextpredlock = (PREDICATELOCK *)
  				SHMQueueNext(&(target->predicateLocks),
  							 &(predlock->targetLink),
  							 offsetof(PREDICATELOCK, targetLink));
  
  			if (predlock->tag.myXact != MySerializableXact
  				&& !RWConflictExists(predlock->tag.myXact, MySerializableXact))
  			{
  				FlagRWConflict(predlock->tag.myXact, MySerializableXact);
  			}
  
  			predlock = nextpredlock;
  		}
  	}
  
  	/* Release locks in reverse order */
  	LWLockRelease(SerializableXactHashLock);
  	for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
  		LWLockRelease(PredicateLockHashPartitionLockByIndex(i));
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  
  /*
   * Flag a rw-dependency between two serializable transactions.
   *
   * The caller is responsible for ensuring that we have a LW lock on
   * the transaction hash table.
   */
  static void
  FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
  {
  	Assert(reader != writer);
  
  	/* First, see if this conflict causes failure. */
  	OnConflict_CheckForSerializationFailure(reader, writer);
  
  	/* Actually do the conflict flagging. */
  	if (reader == OldCommittedSxact)
  		writer->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
  	else if (writer == OldCommittedSxact)
  		reader->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
  	else
  		SetRWConflict(reader, writer);
  }
  
  /*----------------------------------------------------------------------------
   * We are about to add a RW-edge to the dependency graph - check that we don't
   * introduce a dangerous structure by doing so, and abort one of the
   * transactions if so.
   *
   * A serialization failure can only occur if there is a dangerous structure
   * in the dependency graph:
   *
   *		Tin ------> Tpivot ------> Tout
   *			  rw			 rw
   *
   * Furthermore, Tout must commit first.
   *
   * One more optimization is that if Tin is declared READ ONLY (or commits
   * without writing), we can only have a problem if Tout committed before Tin
   * acquired its snapshot.
   *----------------------------------------------------------------------------
   */
  static void
  OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
  										SERIALIZABLEXACT *writer)
  {
  	bool		failure;
  	RWConflict	conflict;
  
  	Assert(LWLockHeldByMe(SerializableXactHashLock));
  
  	failure = false;
  
  	/*------------------------------------------------------------------------
  	 * Check for already-committed writer with rw-conflict out flagged
  	 * (conflict-flag on W means that T2 committed before W):
  	 *
  	 *		R ------> W ------> T2
  	 *			rw		  rw
  	 *
  	 * That is a dangerous structure, so we must abort. (Since the writer
  	 * has already committed, we must be the reader)
  	 *------------------------------------------------------------------------
  	 */
  	if (SxactIsCommitted(writer)
  		&& (SxactHasConflictOut(writer) || SxactHasSummaryConflictOut(writer)))
  		failure = true;
  
  	/*------------------------------------------------------------------------
  	 * Check whether the writer has become a pivot with an out-conflict
  	 * committed transaction (T2), and T2 committed first:
  	 *
  	 *		R ------> W ------> T2
  	 *			rw		  rw
  	 *
  	 * Because T2 must've committed first, there is no anomaly if:
  	 * - the reader committed before T2
  	 * - the writer committed before T2
  	 * - the reader is a READ ONLY transaction and the reader was concurrent
  	 *	 with T2 (= reader acquired its snapshot before T2 committed)
  	 *
  	 * We also handle the case that T2 is prepared but not yet committed
  	 * here. In that case T2 has already checked for conflicts, so if it
  	 * commits first, making the above conflict real, it's too late for it
  	 * to abort.
  	 *------------------------------------------------------------------------
  	 */
  	if (!failure)
  	{
  		if (SxactHasSummaryConflictOut(writer))
  		{
  			failure = true;
  			conflict = NULL;
  		}
  		else
  			conflict = (RWConflict)
  				SHMQueueNext(&writer->outConflicts,
  							 &writer->outConflicts,
  							 offsetof(RWConflictData, outLink));
  		while (conflict)
  		{
  			SERIALIZABLEXACT *t2 = conflict->sxactIn;
  
  			if (SxactIsPrepared(t2)
  				&& (!SxactIsCommitted(reader)
  					|| t2->prepareSeqNo <= reader->commitSeqNo)
  				&& (!SxactIsCommitted(writer)
  					|| t2->prepareSeqNo <= writer->commitSeqNo)
  				&& (!SxactIsReadOnly(reader)
  					|| t2->prepareSeqNo <= reader->SeqNo.lastCommitBeforeSnapshot))
  			{
  				failure = true;
  				break;
  			}
  			conflict = (RWConflict)
  				SHMQueueNext(&writer->outConflicts,
  							 &conflict->outLink,
  							 offsetof(RWConflictData, outLink));
  		}
  	}
  
  	/*------------------------------------------------------------------------
  	 * Check whether the reader has become a pivot with a writer
  	 * that's committed (or prepared):
  	 *
  	 *		T0 ------> R ------> W
  	 *			 rw		   rw
  	 *
  	 * Because W must've committed first for an anomaly to occur, there is no
  	 * anomaly if:
  	 * - T0 committed before the writer
  	 * - T0 is READ ONLY, and overlaps the writer
  	 *------------------------------------------------------------------------
  	 */
  	if (!failure && SxactIsPrepared(writer) && !SxactIsReadOnly(reader))
  	{
  		if (SxactHasSummaryConflictIn(reader))
  		{
  			failure = true;
  			conflict = NULL;
  		}
  		else
  			conflict = (RWConflict)
  				SHMQueueNext(&reader->inConflicts,
  							 &reader->inConflicts,
  							 offsetof(RWConflictData, inLink));
  		while (conflict)
  		{
  			SERIALIZABLEXACT *t0 = conflict->sxactOut;
  
  			if (!SxactIsDoomed(t0)
  				&& (!SxactIsCommitted(t0)
  					|| t0->commitSeqNo >= writer->prepareSeqNo)
  				&& (!SxactIsReadOnly(t0)
  					|| t0->SeqNo.lastCommitBeforeSnapshot >= writer->prepareSeqNo))
  			{
  				failure = true;
  				break;
  			}
  			conflict = (RWConflict)
  				SHMQueueNext(&reader->inConflicts,
  							 &conflict->inLink,
  							 offsetof(RWConflictData, inLink));
  		}
  	}
  
  	if (failure)
  	{
  		/*
  		 * We have to kill a transaction to avoid a possible anomaly from
  		 * occurring. If the writer is us, we can just ereport() to cause a
  		 * transaction abort. Otherwise we flag the writer for termination,
  		 * causing it to abort when it tries to commit. However, if the writer
  		 * is a prepared transaction, already prepared, we can't abort it
  		 * anymore, so we have to kill the reader instead.
  		 */
  		if (MySerializableXact == writer)
  		{
  			LWLockRelease(SerializableXactHashLock);
  			ereport(ERROR,
  					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  					 errmsg("could not serialize access due to read/write dependencies among transactions"),
  					 errdetail_internal("Reason code: Canceled on identification as a pivot, during write."),
  					 errhint("The transaction might succeed if retried.")));
  		}
  		else if (SxactIsPrepared(writer))
  		{
  			LWLockRelease(SerializableXactHashLock);
  
  			/* if we're not the writer, we have to be the reader */
  			Assert(MySerializableXact == reader);
  			ereport(ERROR,
  					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  					 errmsg("could not serialize access due to read/write dependencies among transactions"),
  					 errdetail_internal("Reason code: Canceled on conflict out to pivot %u, during read.", writer->topXid),
  					 errhint("The transaction might succeed if retried.")));
  		}
  		writer->flags |= SXACT_FLAG_DOOMED;
  	}
  }
  
  /*
   * PreCommit_CheckForSerializableConflicts
   *		Check for dangerous structures in a serializable transaction
   *		at commit.
   *
   * We're checking for a dangerous structure as each conflict is recorded.
   * The only way we could have a problem at commit is if this is the "out"
   * side of a pivot, and neither the "in" side nor the pivot has yet
   * committed.
   *
   * If a dangerous structure is found, the pivot (the near conflict) is
   * marked for death, because rolling back another transaction might mean
   * that we flail without ever making progress.  This transaction is
   * committing writes, so letting it commit ensures progress.  If we
   * canceled the far conflict, it might immediately fail again on retry.
   */
  void
  PreCommit_CheckForSerializationFailure(void)
  {
  	RWConflict	nearConflict;
  
  	if (MySerializableXact == InvalidSerializableXact)
  		return;
  
  	Assert(IsolationIsSerializable());
  
  	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  
  	/* Check if someone else has already decided that we need to die */
  	if (SxactIsDoomed(MySerializableXact))
  	{
  		LWLockRelease(SerializableXactHashLock);
  		ereport(ERROR,
  				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  				 errmsg("could not serialize access due to read/write dependencies among transactions"),
  				 errdetail_internal("Reason code: Canceled on identification as a pivot, during commit attempt."),
  				 errhint("The transaction might succeed if retried.")));
  	}
  
  	nearConflict = (RWConflict)
  		SHMQueueNext(&MySerializableXact->inConflicts,
  					 &MySerializableXact->inConflicts,
  					 offsetof(RWConflictData, inLink));
  	while (nearConflict)
  	{
  		if (!SxactIsCommitted(nearConflict->sxactOut)
  			&& !SxactIsDoomed(nearConflict->sxactOut))
  		{
  			RWConflict	farConflict;
  
  			farConflict = (RWConflict)
  				SHMQueueNext(&nearConflict->sxactOut->inConflicts,
  							 &nearConflict->sxactOut->inConflicts,
  							 offsetof(RWConflictData, inLink));
  			while (farConflict)
  			{
  				if (farConflict->sxactOut == MySerializableXact
  					|| (!SxactIsCommitted(farConflict->sxactOut)
  						&& !SxactIsReadOnly(farConflict->sxactOut)
  						&& !SxactIsDoomed(farConflict->sxactOut)))
  				{
  					/*
  					 * Normally, we kill the pivot transaction to make sure we
  					 * make progress if the failing transaction is retried.
  					 * However, we can't kill it if it's already prepared, so
  					 * in that case we commit suicide instead.
  					 */
  					if (SxactIsPrepared(nearConflict->sxactOut))
  					{
  						LWLockRelease(SerializableXactHashLock);
  						ereport(ERROR,
  								(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
  								 errmsg("could not serialize access due to read/write dependencies among transactions"),
  								 errdetail_internal("Reason code: Canceled on commit attempt with conflict in from prepared pivot."),
  								 errhint("The transaction might succeed if retried.")));
  					}
  					nearConflict->sxactOut->flags |= SXACT_FLAG_DOOMED;
  					break;
  				}
  				farConflict = (RWConflict)
  					SHMQueueNext(&nearConflict->sxactOut->inConflicts,
  								 &farConflict->inLink,
  								 offsetof(RWConflictData, inLink));
  			}
  		}
  
  		nearConflict = (RWConflict)
  			SHMQueueNext(&MySerializableXact->inConflicts,
  						 &nearConflict->inLink,
  						 offsetof(RWConflictData, inLink));
  	}
  
  	MySerializableXact->prepareSeqNo = ++(PredXact->LastSxactCommitSeqNo);
  	MySerializableXact->flags |= SXACT_FLAG_PREPARED;
  
  	LWLockRelease(SerializableXactHashLock);
  }
  
  /*------------------------------------------------------------------------*/
  
  /*
   * Two-phase commit support
   */
  
  /*
   * AtPrepare_Locks
   *		Do the preparatory work for a PREPARE: make 2PC state file
   *		records for all predicate locks currently held.
   */
  void
  AtPrepare_PredicateLocks(void)
  {
  	PREDICATELOCK *predlock;
  	SERIALIZABLEXACT *sxact;
  	TwoPhasePredicateRecord record;
  	TwoPhasePredicateXactRecord *xactRecord;
  	TwoPhasePredicateLockRecord *lockRecord;
  
  	sxact = MySerializableXact;
  	xactRecord = &(record.data.xactRecord);
  	lockRecord = &(record.data.lockRecord);
  
  	if (MySerializableXact == InvalidSerializableXact)
  		return;
  
  	/* Generate an xact record for our SERIALIZABLEXACT */
  	record.type = TWOPHASEPREDICATERECORD_XACT;
  	xactRecord->xmin = MySerializableXact->xmin;
  	xactRecord->flags = MySerializableXact->flags;
  
  	/*
  	 * Note that we don't include the list of conflicts in our out in the
  	 * statefile, because new conflicts can be added even after the
  	 * transaction prepares. We'll just make a conservative assumption during
  	 * recovery instead.
  	 */
  
  	RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
  						   &record, sizeof(record));
  
  	/*
  	 * Generate a lock record for each lock.
  	 *
  	 * To do this, we need to walk the predicate lock list in our sxact rather
  	 * than using the local predicate lock table because the latter is not
  	 * guaranteed to be accurate.
  	 */
  	LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
  
  	predlock = (PREDICATELOCK *)
  		SHMQueueNext(&(sxact->predicateLocks),
  					 &(sxact->predicateLocks),
  					 offsetof(PREDICATELOCK, xactLink));
  
  	while (predlock != NULL)
  	{
  		record.type = TWOPHASEPREDICATERECORD_LOCK;
  		lockRecord->target = predlock->tag.myTarget->tag;
  
  		RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
  							   &record, sizeof(record));
  
  		predlock = (PREDICATELOCK *)
  			SHMQueueNext(&(sxact->predicateLocks),
  						 &(predlock->xactLink),
  						 offsetof(PREDICATELOCK, xactLink));
  	}
  
  	LWLockRelease(SerializablePredicateLockListLock);
  }
  
  /*
   * PostPrepare_Locks
   *		Clean up after successful PREPARE. Unlike the non-predicate
   *		lock manager, we do not need to transfer locks to a dummy
   *		PGPROC because our SERIALIZABLEXACT will stay around
   *		anyway. We only need to clean up our local state.
   */
  void
  PostPrepare_PredicateLocks(TransactionId xid)
  {
  	if (MySerializableXact == InvalidSerializableXact)
  		return;
  
  	Assert(SxactIsPrepared(MySerializableXact));
  
  	MySerializableXact->pid = 0;
  
  	hash_destroy(LocalPredicateLockHash);
  	LocalPredicateLockHash = NULL;
  
  	MySerializableXact = InvalidSerializableXact;
  	MyXactDidWrite = false;
  }
  
  /*
   * PredicateLockTwoPhaseFinish
   *		Release a prepared transaction's predicate locks once it
   *		commits or aborts.
   */
  void
  PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit)
  {
  	SERIALIZABLEXID *sxid;
  	SERIALIZABLEXIDTAG sxidtag;
  
  	sxidtag.xid = xid;
  
  	LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  	sxid = (SERIALIZABLEXID *)
  		hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
  	LWLockRelease(SerializableXactHashLock);
  
  	/* xid will not be found if it wasn't a serializable transaction */
  	if (sxid == NULL)
  		return;
  
  	/* Release its locks */
  	MySerializableXact = sxid->myXact;
  	MyXactDidWrite = true;		/* conservatively assume that we wrote
  								 * something */
  	ReleasePredicateLocks(isCommit);
  }
  
  /*
   * Re-acquire a predicate lock belonging to a transaction that was prepared.
   */
  void
  predicatelock_twophase_recover(TransactionId xid, uint16 info,
  							   void *recdata, uint32 len)
  {
  	TwoPhasePredicateRecord *record;
  
  	Assert(len == sizeof(TwoPhasePredicateRecord));
  
  	record = (TwoPhasePredicateRecord *) recdata;
  
  	Assert((record->type == TWOPHASEPREDICATERECORD_XACT) ||
  		   (record->type == TWOPHASEPREDICATERECORD_LOCK));
  
  	if (record->type == TWOPHASEPREDICATERECORD_XACT)
  	{
  		/* Per-transaction record. Set up a SERIALIZABLEXACT. */
  		TwoPhasePredicateXactRecord *xactRecord;
  		SERIALIZABLEXACT *sxact;
  		SERIALIZABLEXID *sxid;
  		SERIALIZABLEXIDTAG sxidtag;
  		bool		found;
  
  		xactRecord = (TwoPhasePredicateXactRecord *) &record->data.xactRecord;
  
  		LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
  		sxact = CreatePredXact();
  		if (!sxact)
  			ereport(ERROR,
  					(errcode(ERRCODE_OUT_OF_MEMORY),
  					 errmsg("out of shared memory")));
  
  		/* vxid for a prepared xact is InvalidBackendId/xid; no pid */
  		sxact->vxid.backendId = InvalidBackendId;
  		sxact->vxid.localTransactionId = (LocalTransactionId) xid;
  		sxact->pid = 0;
  
  		/* a prepared xact hasn't committed yet */
  		sxact->prepareSeqNo = RecoverySerCommitSeqNo;
  		sxact->commitSeqNo = InvalidSerCommitSeqNo;
  		sxact->finishedBefore = InvalidTransactionId;
  
  		sxact->SeqNo.lastCommitBeforeSnapshot = RecoverySerCommitSeqNo;
  
  		/*
  		 * Don't need to track this; no transactions running at the time the
  		 * recovered xact started are still active, except possibly other
  		 * prepared xacts and we don't care whether those are RO_SAFE or not.
  		 */
  		SHMQueueInit(&(sxact->possibleUnsafeConflicts));
  
  		SHMQueueInit(&(sxact->predicateLocks));
  		SHMQueueElemInit(&(sxact->finishedLink));
  
  		sxact->topXid = xid;
  		sxact->xmin = xactRecord->xmin;
  		sxact->flags = xactRecord->flags;
  		Assert(SxactIsPrepared(sxact));
  		if (!SxactIsReadOnly(sxact))
  		{
  			++(PredXact->WritableSxactCount);
  			Assert(PredXact->WritableSxactCount <=
  				   (MaxBackends + max_prepared_xacts));
  		}
  
  		/*
  		 * We don't know whether the transaction had any conflicts or not, so
  		 * we'll conservatively assume that it had both a conflict in and a
  		 * conflict out, and represent that with the summary conflict flags.
  		 */
  		SHMQueueInit(&(sxact->outConflicts));
  		SHMQueueInit(&(sxact->inConflicts));
  		sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
  		sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
  
  		/* Register the transaction's xid */
  		sxidtag.xid = xid;
  		sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
  											   &sxidtag,
  											   HASH_ENTER, &found);
  		Assert(sxid != NULL);
  		Assert(!found);
  		sxid->myXact = (SERIALIZABLEXACT *) sxact;
  
  		/*
  		 * Update global xmin. Note that this is a special case compared to
  		 * registering a normal transaction, because the global xmin might go
  		 * backwards. That's OK, because until recovery is over we're not
  		 * going to complete any transactions or create any non-prepared
  		 * transactions, so there's no danger of throwing away.
  		 */
  		if ((!TransactionIdIsValid(PredXact->SxactGlobalXmin)) ||
  			(TransactionIdFollows(PredXact->SxactGlobalXmin, sxact->xmin)))
  		{
  			PredXact->SxactGlobalXmin = sxact->xmin;
  			PredXact->SxactGlobalXminCount = 1;
  			OldSerXidSetActiveSerXmin(sxact->xmin);
  		}
  		else if (TransactionIdEquals(sxact->xmin, PredXact->SxactGlobalXmin))
  		{
  			Assert(PredXact->SxactGlobalXminCount > 0);
  			PredXact->SxactGlobalXminCount++;
  		}
  
  		LWLockRelease(SerializableXactHashLock);
  	}
  	else if (record->type == TWOPHASEPREDICATERECORD_LOCK)
  	{
  		/* Lock record. Recreate the PREDICATELOCK */
  		TwoPhasePredicateLockRecord *lockRecord;
  		SERIALIZABLEXID *sxid;
  		SERIALIZABLEXACT *sxact;
  		SERIALIZABLEXIDTAG sxidtag;
  		uint32		targettaghash;
  
  		lockRecord = (TwoPhasePredicateLockRecord *) &record->data.lockRecord;
  		targettaghash = PredicateLockTargetTagHashCode(&lockRecord->target);
  
  		LWLockAcquire(SerializableXactHashLock, LW_SHARED);
  		sxidtag.xid = xid;
  		sxid = (SERIALIZABLEXID *)
  			hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
  		LWLockRelease(SerializableXactHashLock);
  
  		Assert(sxid != NULL);
  		sxact = sxid->myXact;
  		Assert(sxact != InvalidSerializableXact);
  
  		CreatePredicateLock(&lockRecord->target, targettaghash, sxact);
  	}
  }
  