text: /*-------------------------------------------------------------------------
   *
   * hashpage.c
   *	  Hash table page management code for the Postgres hash access method
   *
   * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/access/hash/hashpage.c
   *
   * NOTES
   *	  Postgres hash pages look like ordinary relation pages.  The opaque
   *	  data at high addresses includes information about the page including
   *	  whether a page is an overflow page or a true bucket, the bucket
   *	  number, and the block numbers of the preceding and following pages
   *	  in the same bucket.
   *
   *	  The first page in a hash relation, page zero, is special -- it stores
   *	  information describing the hash table; it is referred to as the
   *	  "meta page." Pages one and higher store the actual data.
   *
   *	  There are also bitmap pages, which are not manipulated here;
   *	  see hashovfl.c.
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include "access/hash.h"
  #include "access/hash_xlog.h"
  #include "miscadmin.h"
  #include "storage/lmgr.h"
  #include "storage/smgr.h"
  #include "storage/predicate.h"
  
  
  static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
  					uint32 nblocks);
  static void _hash_splitbucket(Relation rel, Buffer metabuf,
  				  Bucket obucket, Bucket nbucket,
  				  Buffer obuf,
  				  Buffer nbuf,
  				  HTAB *htab,
  				  uint32 maxbucket,
  				  uint32 highmask, uint32 lowmask);
  static void log_split_page(Relation rel, Buffer buf);
  
  
  /*
   *	_hash_getbuf() -- Get a buffer by block number for read or write.
   *
   *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
   *		'flags' is a bitwise OR of the allowed page types.
   *
   *		This must be used only to fetch pages that are expected to be valid
   *		already.  _hash_checkpage() is applied using the given flags.
   *
   *		When this routine returns, the appropriate lock is set on the
   *		requested buffer and its reference count has been incremented
   *		(ie, the buffer is "locked and pinned").
   *
   *		P_NEW is disallowed because this routine can only be used
   *		to access pages that are known to be before the filesystem EOF.
   *		Extending the index should be done with _hash_getnewbuf.
   */
  Buffer
  _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
  {
  	Buffer		buf;
  
  	if (blkno == P_NEW)
  		elog(ERROR, "hash AM does not use P_NEW");
  
  	buf = ReadBuffer(rel, blkno);
  
  	if (access != HASH_NOLOCK)
  		LockBuffer(buf, access);
  
  	/* ref count and lock type are correct */
  
  	_hash_checkpage(rel, buf, flags);
  
  	return buf;
  }
  
  /*
   * _hash_getbuf_with_condlock_cleanup() -- Try to get a buffer for cleanup.
   *
   *		We read the page and try to acquire a cleanup lock.  If we get it,
   *		we return the buffer; otherwise, we return InvalidBuffer.
   */
  Buffer
  _hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
  {
  	Buffer		buf;
  
  	if (blkno == P_NEW)
  		elog(ERROR, "hash AM does not use P_NEW");
  
  	buf = ReadBuffer(rel, blkno);
  
  	if (!ConditionalLockBufferForCleanup(buf))
  	{
  		ReleaseBuffer(buf);
  		return InvalidBuffer;
  	}
  
  	/* ref count and lock type are correct */
  
  	_hash_checkpage(rel, buf, flags);
  
  	return buf;
  }
  
  /*
   *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
   *
   *		This must be used only to fetch pages that are known to be before
   *		the index's filesystem EOF, but are to be filled from scratch.
   *		_hash_pageinit() is applied automatically.  Otherwise it has
   *		effects similar to _hash_getbuf() with access = HASH_WRITE.
   *
   *		When this routine returns, a write lock is set on the
   *		requested buffer and its reference count has been incremented
   *		(ie, the buffer is "locked and pinned").
   *
   *		P_NEW is disallowed because this routine can only be used
   *		to access pages that are known to be before the filesystem EOF.
   *		Extending the index should be done with _hash_getnewbuf.
   */
  Buffer
  _hash_getinitbuf(Relation rel, BlockNumber blkno)
  {
  	Buffer		buf;
  
  	if (blkno == P_NEW)
  		elog(ERROR, "hash AM does not use P_NEW");
  
  	buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO_AND_LOCK,
  							 NULL);
  
  	/* ref count and lock type are correct */
  
  	/* initialize the page */
  	_hash_pageinit(BufferGetPage(buf), BufferGetPageSize(buf));
  
  	return buf;
  }
  
  /*
   *	_hash_initbuf() -- Get and initialize a buffer by bucket number.
   */
  void
  _hash_initbuf(Buffer buf, uint32 max_bucket, uint32 num_bucket, uint32 flag,
  			  bool initpage)
  {
  	HashPageOpaque pageopaque;
  	Page		page;
  
  	page = BufferGetPage(buf);
  
  	/* initialize the page */
  	if (initpage)
  		_hash_pageinit(page, BufferGetPageSize(buf));
  
  	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
  
  	/*
  	 * Set hasho_prevblkno with current hashm_maxbucket. This value will be
  	 * used to validate cached HashMetaPageData. See
  	 * _hash_getbucketbuf_from_hashkey().
  	 */
  	pageopaque->hasho_prevblkno = max_bucket;
  	pageopaque->hasho_nextblkno = InvalidBlockNumber;
  	pageopaque->hasho_bucket = num_bucket;
  	pageopaque->hasho_flag = flag;
  	pageopaque->hasho_page_id = HASHO_PAGE_ID;
  }
  
  /*
   *	_hash_getnewbuf() -- Get a new page at the end of the index.
   *
   *		This has the same API as _hash_getinitbuf, except that we are adding
   *		a page to the index, and hence expect the page to be past the
   *		logical EOF.  (However, we have to support the case where it isn't,
   *		since a prior try might have crashed after extending the filesystem
   *		EOF but before updating the metapage to reflect the added page.)
   *
   *		It is caller's responsibility to ensure that only one process can
   *		extend the index at a time.  In practice, this function is called
   *		only while holding write lock on the metapage, because adding a page
   *		is always associated with an update of metapage data.
   */
  Buffer
  _hash_getnewbuf(Relation rel, BlockNumber blkno, ForkNumber forkNum)
  {
  	BlockNumber nblocks = RelationGetNumberOfBlocksInFork(rel, forkNum);
  	Buffer		buf;
  
  	if (blkno == P_NEW)
  		elog(ERROR, "hash AM does not use P_NEW");
  	if (blkno > nblocks)
  		elog(ERROR, "access to noncontiguous page in hash index \"%s\"",
  			 RelationGetRelationName(rel));
  
  	/* smgr insists we use P_NEW to extend the relation */
  	if (blkno == nblocks)
  	{
  		buf = ReadBufferExtended(rel, forkNum, P_NEW, RBM_NORMAL, NULL);
  		if (BufferGetBlockNumber(buf) != blkno)
  			elog(ERROR, "unexpected hash relation size: %u, should be %u",
  				 BufferGetBlockNumber(buf), blkno);
  		LockBuffer(buf, HASH_WRITE);
  	}
  	else
  	{
  		buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO_AND_LOCK,
  								 NULL);
  	}
  
  	/* ref count and lock type are correct */
  
  	/* initialize the page */
  	_hash_pageinit(BufferGetPage(buf), BufferGetPageSize(buf));
  
  	return buf;
  }
  
  /*
   *	_hash_getbuf_with_strategy() -- Get a buffer with nondefault strategy.
   *
   *		This is identical to _hash_getbuf() but also allows a buffer access
   *		strategy to be specified.  We use this for VACUUM operations.
   */
  Buffer
  _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
  						   int access, int flags,
  						   BufferAccessStrategy bstrategy)
  {
  	Buffer		buf;
  
  	if (blkno == P_NEW)
  		elog(ERROR, "hash AM does not use P_NEW");
  
  	buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, bstrategy);
  
  	if (access != HASH_NOLOCK)
  		LockBuffer(buf, access);
  
  	/* ref count and lock type are correct */
  
  	_hash_checkpage(rel, buf, flags);
  
  	return buf;
  }
  
  /*
   *	_hash_relbuf() -- release a locked buffer.
   *
   * Lock and pin (refcount) are both dropped.
   */
  void
  _hash_relbuf(Relation rel, Buffer buf)
  {
  	UnlockReleaseBuffer(buf);
  }
  
  /*
   *	_hash_dropbuf() -- release an unlocked buffer.
   *
   * This is used to unpin a buffer on which we hold no lock.
   */
  void
  _hash_dropbuf(Relation rel, Buffer buf)
  {
  	ReleaseBuffer(buf);
  }
  
  /*
   *	_hash_dropscanbuf() -- release buffers used in scan.
   *
   * This routine unpins the buffers used during scan on which we
   * hold no lock.
   */
  void
  _hash_dropscanbuf(Relation rel, HashScanOpaque so)
  {
  	/* release pin we hold on primary bucket page */
  	if (BufferIsValid(so->hashso_bucket_buf) &&
  		so->hashso_bucket_buf != so->currPos.buf)
  		_hash_dropbuf(rel, so->hashso_bucket_buf);
  	so->hashso_bucket_buf = InvalidBuffer;
  
  	/* release pin we hold on primary bucket page  of bucket being split */
  	if (BufferIsValid(so->hashso_split_bucket_buf) &&
  		so->hashso_split_bucket_buf != so->currPos.buf)
  		_hash_dropbuf(rel, so->hashso_split_bucket_buf);
  	so->hashso_split_bucket_buf = InvalidBuffer;
  
  	/* release any pin we still hold */
  	if (BufferIsValid(so->currPos.buf))
  		_hash_dropbuf(rel, so->currPos.buf);
  	so->currPos.buf = InvalidBuffer;
  
  	/* reset split scan */
  	so->hashso_buc_populated = false;
  	so->hashso_buc_split = false;
  }
  
  
  /*
   *	_hash_init() -- Initialize the metadata page of a hash index,
   *				the initial buckets, and the initial bitmap page.
   *
   * The initial number of buckets is dependent on num_tuples, an estimate
   * of the number of tuples to be loaded into the index initially.  The
   * chosen number of buckets is returned.
   *
   * We are fairly cavalier about locking here, since we know that no one else
   * could be accessing this index.  In particular the rule about not holding
   * multiple buffer locks is ignored.
   */
  uint32
  _hash_init(Relation rel, double num_tuples, ForkNumber forkNum)
  {
  	Buffer		metabuf;
  	Buffer		buf;
  	Buffer		bitmapbuf;
  	Page		pg;
  	HashMetaPage metap;
  	RegProcedure procid;
  	int32		data_width;
  	int32		item_width;
  	int32		ffactor;
  	uint32		num_buckets;
  	uint32		i;
  	bool		use_wal;
  
  	/* safety check */
  	if (RelationGetNumberOfBlocksInFork(rel, forkNum) != 0)
  		elog(ERROR, "cannot initialize non-empty hash index \"%s\"",
  			 RelationGetRelationName(rel));
  
  	/*
  	 * WAL log creation of pages if the relation is persistent, or this is the
  	 * init fork.  Init forks for unlogged relations always need to be WAL
  	 * logged.
  	 */
  	use_wal = RelationNeedsWAL(rel) || forkNum == INIT_FORKNUM;
  
  	/*
  	 * Determine the target fill factor (in tuples per bucket) for this index.
  	 * The idea is to make the fill factor correspond to pages about as full
  	 * as the user-settable fillfactor parameter says.  We can compute it
  	 * exactly since the index datatype (i.e. uint32 hash key) is fixed-width.
  	 */
  	data_width = sizeof(uint32);
  	item_width = MAXALIGN(sizeof(IndexTupleData)) + MAXALIGN(data_width) +
  		sizeof(ItemIdData);		/* include the line pointer */
  	ffactor = RelationGetTargetPageUsage(rel, HASH_DEFAULT_FILLFACTOR) / item_width;
  	/* keep to a sane range */
  	if (ffactor < 10)
  		ffactor = 10;
  
  	procid = index_getprocid(rel, 1, HASHSTANDARD_PROC);
  
  	/*
  	 * We initialize the metapage, the first N bucket pages, and the first
  	 * bitmap page in sequence, using _hash_getnewbuf to cause smgrextend()
  	 * calls to occur.  This ensures that the smgr level has the right idea of
  	 * the physical index length.
  	 *
  	 * Critical section not required, because on error the creation of the
  	 * whole relation will be rolled back.
  	 */
  	metabuf = _hash_getnewbuf(rel, HASH_METAPAGE, forkNum);
  	_hash_init_metabuffer(metabuf, num_tuples, procid, ffactor, false);
  	MarkBufferDirty(metabuf);
  
  	pg = BufferGetPage(metabuf);
  	metap = HashPageGetMeta(pg);
  
  	/* XLOG stuff */
  	if (use_wal)
  	{
  		xl_hash_init_meta_page xlrec;
  		XLogRecPtr	recptr;
  
  		xlrec.num_tuples = num_tuples;
  		xlrec.procid = metap->hashm_procid;
  		xlrec.ffactor = metap->hashm_ffactor;
  
  		XLogBeginInsert();
  		XLogRegisterData((char *) &xlrec, SizeOfHashInitMetaPage);
  		XLogRegisterBuffer(0, metabuf, REGBUF_WILL_INIT | REGBUF_STANDARD);
  
  		recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_INIT_META_PAGE);
  
  		PageSetLSN(BufferGetPage(metabuf), recptr);
  	}
  
  	num_buckets = metap->hashm_maxbucket + 1;
  
  	/*
  	 * Release buffer lock on the metapage while we initialize buckets.
  	 * Otherwise, we'll be in interrupt holdoff and the CHECK_FOR_INTERRUPTS
  	 * won't accomplish anything.  It's a bad idea to hold buffer locks for
  	 * long intervals in any case, since that can block the bgwriter.
  	 */
  	LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
  
  	/*
  	 * Initialize and WAL Log the first N buckets
  	 */
  	for (i = 0; i < num_buckets; i++)
  	{
  		BlockNumber blkno;
  
  		/* Allow interrupts, in case N is huge */
  		CHECK_FOR_INTERRUPTS();
  
  		blkno = BUCKET_TO_BLKNO(metap, i);
  		buf = _hash_getnewbuf(rel, blkno, forkNum);
  		_hash_initbuf(buf, metap->hashm_maxbucket, i, LH_BUCKET_PAGE, false);
  		MarkBufferDirty(buf);
  
  		if (use_wal)
  			log_newpage(&rel->rd_node,
  						forkNum,
  						blkno,
  						BufferGetPage(buf),
  						true);
  		_hash_relbuf(rel, buf);
  	}
  
  	/* Now reacquire buffer lock on metapage */
  	LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
  
  	/*
  	 * Initialize bitmap page
  	 */
  	bitmapbuf = _hash_getnewbuf(rel, num_buckets + 1, forkNum);
  	_hash_initbitmapbuffer(bitmapbuf, metap->hashm_bmsize, false);
  	MarkBufferDirty(bitmapbuf);
  
  	/* add the new bitmap page to the metapage's list of bitmaps */
  	/* metapage already has a write lock */
  	if (metap->hashm_nmaps >= HASH_MAX_BITMAPS)
  		ereport(ERROR,
  				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
  				 errmsg("out of overflow pages in hash index \"%s\"",
  						RelationGetRelationName(rel))));
  
  	metap->hashm_mapp[metap->hashm_nmaps] = num_buckets + 1;
  
  	metap->hashm_nmaps++;
  	MarkBufferDirty(metabuf);
  
  	/* XLOG stuff */
  	if (use_wal)
  	{
  		xl_hash_init_bitmap_page xlrec;
  		XLogRecPtr	recptr;
  
  		xlrec.bmsize = metap->hashm_bmsize;
  
  		XLogBeginInsert();
  		XLogRegisterData((char *) &xlrec, SizeOfHashInitBitmapPage);
  		XLogRegisterBuffer(0, bitmapbuf, REGBUF_WILL_INIT);
  
  		/*
  		 * This is safe only because nobody else can be modifying the index at
  		 * this stage; it's only visible to the transaction that is creating
  		 * it.
  		 */
  		XLogRegisterBuffer(1, metabuf, REGBUF_STANDARD);
  
  		recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_INIT_BITMAP_PAGE);
  
  		PageSetLSN(BufferGetPage(bitmapbuf), recptr);
  		PageSetLSN(BufferGetPage(metabuf), recptr);
  	}
  
  	/* all done */
  	_hash_relbuf(rel, bitmapbuf);
  	_hash_relbuf(rel, metabuf);
  
  	return num_buckets;
  }
  
  /*
   *	_hash_init_metabuffer() -- Initialize the metadata page of a hash index.
   */
  void
  _hash_init_metabuffer(Buffer buf, double num_tuples, RegProcedure procid,
  					  uint16 ffactor, bool initpage)
  {
  	HashMetaPage metap;
  	HashPageOpaque pageopaque;
  	Page		page;
  	double		dnumbuckets;
  	uint32		num_buckets;
  	uint32		spare_index;
  	uint32		i;
  
  	/*
  	 * Choose the number of initial bucket pages to match the fill factor
  	 * given the estimated number of tuples.  We round up the result to the
  	 * total number of buckets which has to be allocated before using its
  	 * _hashm_spare element. However always force at least 2 bucket pages. The
  	 * upper limit is determined by considerations explained in
  	 * _hash_expandtable().
  	 */
  	dnumbuckets = num_tuples / ffactor;
  	if (dnumbuckets <= 2.0)
  		num_buckets = 2;
  	else if (dnumbuckets >= (double) 0x40000000)
  		num_buckets = 0x40000000;
  	else
  		num_buckets = _hash_get_totalbuckets(_hash_spareindex(dnumbuckets));
  
  	spare_index = _hash_spareindex(num_buckets);
  	Assert(spare_index < HASH_MAX_SPLITPOINTS);
  
  	page = BufferGetPage(buf);
  	if (initpage)
  		_hash_pageinit(page, BufferGetPageSize(buf));
  
  	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
  	pageopaque->hasho_prevblkno = InvalidBlockNumber;
  	pageopaque->hasho_nextblkno = InvalidBlockNumber;
  	pageopaque->hasho_bucket = -1;
  	pageopaque->hasho_flag = LH_META_PAGE;
  	pageopaque->hasho_page_id = HASHO_PAGE_ID;
  
  	metap = HashPageGetMeta(page);
  
  	metap->hashm_magic = HASH_MAGIC;
  	metap->hashm_version = HASH_VERSION;
  	metap->hashm_ntuples = 0;
  	metap->hashm_nmaps = 0;
  	metap->hashm_ffactor = ffactor;
  	metap->hashm_bsize = HashGetMaxBitmapSize(page);
  	/* find largest bitmap array size that will fit in page size */
  	for (i = _hash_log2(metap->hashm_bsize); i > 0; --i)
  	{
  		if ((1 << i) <= metap->hashm_bsize)
  			break;
  	}
  	Assert(i > 0);
  	metap->hashm_bmsize = 1 << i;
  	metap->hashm_bmshift = i + BYTE_TO_BIT;
  	Assert((1 << BMPG_SHIFT(metap)) == (BMPG_MASK(metap) + 1));
  
  	/*
  	 * Label the index with its primary hash support function's OID.  This is
  	 * pretty useless for normal operation (in fact, hashm_procid is not used
  	 * anywhere), but it might be handy for forensic purposes so we keep it.
  	 */
  	metap->hashm_procid = procid;
  
  	/*
  	 * We initialize the index with N buckets, 0 .. N-1, occupying physical
  	 * blocks 1 to N.  The first freespace bitmap page is in block N+1.
  	 */
  	metap->hashm_maxbucket = num_buckets - 1;
  
  	/*
  	 * Set highmask as next immediate ((2 ^ x) - 1), which should be
  	 * sufficient to cover num_buckets.
  	 */
  	metap->hashm_highmask = (1 << (_hash_log2(num_buckets + 1))) - 1;
  	metap->hashm_lowmask = (metap->hashm_highmask >> 1);
  
  	MemSet(metap->hashm_spares, 0, sizeof(metap->hashm_spares));
  	MemSet(metap->hashm_mapp, 0, sizeof(metap->hashm_mapp));
  
  	/* Set up mapping for one spare page after the initial splitpoints */
  	metap->hashm_spares[spare_index] = 1;
  	metap->hashm_ovflpoint = spare_index;
  	metap->hashm_firstfree = 0;
  
  	/*
  	 * Set pd_lower just past the end of the metadata.  This is essential,
  	 * because without doing so, metadata will be lost if xlog.c compresses
  	 * the page.
  	 */
  	((PageHeader) page)->pd_lower =
  		((char *) metap + sizeof(HashMetaPageData)) - (char *) page;
  }
  
  /*
   *	_hash_pageinit() -- Initialize a new hash index page.
   */
  void
  _hash_pageinit(Page page, Size size)
  {
  	PageInit(page, size, sizeof(HashPageOpaqueData));
  }
  
  /*
   * Attempt to expand the hash table by creating one new bucket.
   *
   * This will silently do nothing if we don't get cleanup lock on old or
   * new bucket.
   *
   * Complete the pending splits and remove the tuples from old bucket,
   * if there are any left over from the previous split.
   *
   * The caller must hold a pin, but no lock, on the metapage buffer.
   * The buffer is returned in the same state.
   */
  void
  _hash_expandtable(Relation rel, Buffer metabuf)
  {
  	HashMetaPage metap;
  	Bucket		old_bucket;
  	Bucket		new_bucket;
  	uint32		spare_ndx;
  	BlockNumber start_oblkno;
  	BlockNumber start_nblkno;
  	Buffer		buf_nblkno;
  	Buffer		buf_oblkno;
  	Page		opage;
  	Page		npage;
  	HashPageOpaque oopaque;
  	HashPageOpaque nopaque;
  	uint32		maxbucket;
  	uint32		highmask;
  	uint32		lowmask;
  	bool		metap_update_masks = false;
  	bool		metap_update_splitpoint = false;
  
  restart_expand:
  
  	/*
  	 * Write-lock the meta page.  It used to be necessary to acquire a
  	 * heavyweight lock to begin a split, but that is no longer required.
  	 */
  	LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
  
  	_hash_checkpage(rel, metabuf, LH_META_PAGE);
  	metap = HashPageGetMeta(BufferGetPage(metabuf));
  
  	/*
  	 * Check to see if split is still needed; someone else might have already
  	 * done one while we waited for the lock.
  	 *
  	 * Make sure this stays in sync with _hash_doinsert()
  	 */
  	if (metap->hashm_ntuples <=
  		(double) metap->hashm_ffactor * (metap->hashm_maxbucket + 1))
  		goto fail;
  
  	/*
  	 * Can't split anymore if maxbucket has reached its maximum possible
  	 * value.
  	 *
  	 * Ideally we'd allow bucket numbers up to UINT_MAX-1 (no higher because
  	 * the calculation maxbucket+1 mustn't overflow).  Currently we restrict
  	 * to half that because of overflow looping in _hash_log2() and
  	 * insufficient space in hashm_spares[].  It's moot anyway because an
  	 * index with 2^32 buckets would certainly overflow BlockNumber and hence
  	 * _hash_alloc_buckets() would fail, but if we supported buckets smaller
  	 * than a disk block then this would be an independent constraint.
  	 *
  	 * If you change this, see also the maximum initial number of buckets in
  	 * _hash_init().
  	 */
  	if (metap->hashm_maxbucket >= (uint32) 0x7FFFFFFE)
  		goto fail;
  
  	/*
  	 * Determine which bucket is to be split, and attempt to take cleanup lock
  	 * on the old bucket.  If we can't get the lock, give up.
  	 *
  	 * The cleanup lock protects us not only against other backends, but
  	 * against our own backend as well.
  	 *
  	 * The cleanup lock is mainly to protect the split from concurrent
  	 * inserts. See src/backend/access/hash/README, Lock Definitions for
  	 * further details.  Due to this locking restriction, if there is any
  	 * pending scan, the split will give up which is not good, but harmless.
  	 */
  	new_bucket = metap->hashm_maxbucket + 1;
  
  	old_bucket = (new_bucket & metap->hashm_lowmask);
  
  	start_oblkno = BUCKET_TO_BLKNO(metap, old_bucket);
  
  	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
  	if (!buf_oblkno)
  		goto fail;
  
  	opage = BufferGetPage(buf_oblkno);
  	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
  
  	/*
  	 * We want to finish the split from a bucket as there is no apparent
  	 * benefit by not doing so and it will make the code complicated to finish
  	 * the split that involves multiple buckets considering the case where new
  	 * split also fails.  We don't need to consider the new bucket for
  	 * completing the split here as it is not possible that a re-split of new
  	 * bucket starts when there is still a pending split from old bucket.
  	 */
  	if (H_BUCKET_BEING_SPLIT(oopaque))
  	{
  		/*
  		 * Copy bucket mapping info now; refer the comment in code below where
  		 * we copy this information before calling _hash_splitbucket to see
  		 * why this is okay.
  		 */
  		maxbucket = metap->hashm_maxbucket;
  		highmask = metap->hashm_highmask;
  		lowmask = metap->hashm_lowmask;
  
  		/*
  		 * Release the lock on metapage and old_bucket, before completing the
  		 * split.
  		 */
  		LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
  		LockBuffer(buf_oblkno, BUFFER_LOCK_UNLOCK);
  
  		_hash_finish_split(rel, metabuf, buf_oblkno, old_bucket, maxbucket,
  						   highmask, lowmask);
  
  		/* release the pin on old buffer and retry for expand. */
  		_hash_dropbuf(rel, buf_oblkno);
  
  		goto restart_expand;
  	}
  
  	/*
  	 * Clean the tuples remained from the previous split.  This operation
  	 * requires cleanup lock and we already have one on the old bucket, so
  	 * let's do it. We also don't want to allow further splits from the bucket
  	 * till the garbage of previous split is cleaned.  This has two
  	 * advantages; first, it helps in avoiding the bloat due to garbage and
  	 * second is, during cleanup of bucket, we are always sure that the
  	 * garbage tuples belong to most recently split bucket.  On the contrary,
  	 * if we allow cleanup of bucket after meta page is updated to indicate
  	 * the new split and before the actual split, the cleanup operation won't
  	 * be able to decide whether the tuple has been moved to the newly created
  	 * bucket and ended up deleting such tuples.
  	 */
  	if (H_NEEDS_SPLIT_CLEANUP(oopaque))
  	{
  		/*
  		 * Copy bucket mapping info now; refer to the comment in code below
  		 * where we copy this information before calling _hash_splitbucket to
  		 * see why this is okay.
  		 */
  		maxbucket = metap->hashm_maxbucket;
  		highmask = metap->hashm_highmask;
  		lowmask = metap->hashm_lowmask;
  
  		/* Release the metapage lock. */
  		LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
  
  		hashbucketcleanup(rel, old_bucket, buf_oblkno, start_oblkno, NULL,
  						  maxbucket, highmask, lowmask, NULL, NULL, true,
  						  NULL, NULL);
  
  		_hash_dropbuf(rel, buf_oblkno);
  
  		goto restart_expand;
  	}
  
  	/*
  	 * There shouldn't be any active scan on new bucket.
  	 *
  	 * Note: it is safe to compute the new bucket's blkno here, even though we
  	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
  	 * the current value of hashm_spares[hashm_ovflpoint] correctly shows
  	 * where we are going to put a new splitpoint's worth of buckets.
  	 */
  	start_nblkno = BUCKET_TO_BLKNO(metap, new_bucket);
  
  	/*
  	 * If the split point is increasing we need to allocate a new batch of
  	 * bucket pages.
  	 */
  	spare_ndx = _hash_spareindex(new_bucket + 1);
  	if (spare_ndx > metap->hashm_ovflpoint)
  	{
  		uint32		buckets_to_add;
  
  		Assert(spare_ndx == metap->hashm_ovflpoint + 1);
  
  		/*
  		 * We treat allocation of buckets as a separate WAL-logged action.
  		 * Even if we fail after this operation, won't leak bucket pages;
  		 * rather, the next split will consume this space. In any case, even
  		 * without failure we don't use all the space in one split operation.
  		 */
  		buckets_to_add = _hash_get_totalbuckets(spare_ndx) - new_bucket;
  		if (!_hash_alloc_buckets(rel, start_nblkno, buckets_to_add))
  		{
  			/* can't split due to BlockNumber overflow */
  			_hash_relbuf(rel, buf_oblkno);
  			goto fail;
  		}
  	}
  
  	/*
  	 * Physically allocate the new bucket's primary page.  We want to do this
  	 * before changing the metapage's mapping info, in case we can't get the
  	 * disk space.  Ideally, we don't need to check for cleanup lock on new
  	 * bucket as no other backend could find this bucket unless meta page is
  	 * updated.  However, it is good to be consistent with old bucket locking.
  	 */
  	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
  	if (!IsBufferCleanupOK(buf_nblkno))
  	{
  		_hash_relbuf(rel, buf_oblkno);
  		_hash_relbuf(rel, buf_nblkno);
  		goto fail;
  	}
  
  	/*
  	 * Since we are scribbling on the pages in the shared buffers, establish a
  	 * critical section.  Any failure in this next code leaves us with a big
  	 * problem: the metapage is effectively corrupt but could get written back
  	 * to disk.
  	 */
  	START_CRIT_SECTION();
  
  	/*
  	 * Okay to proceed with split.  Update the metapage bucket mapping info.
  	 */
  	metap->hashm_maxbucket = new_bucket;
  
  	if (new_bucket > metap->hashm_highmask)
  	{
  		/* Starting a new doubling */
  		metap->hashm_lowmask = metap->hashm_highmask;
  		metap->hashm_highmask = new_bucket | metap->hashm_lowmask;
  		metap_update_masks = true;
  	}
  
  	/*
  	 * If the split point is increasing we need to adjust the hashm_spares[]
  	 * array and hashm_ovflpoint so that future overflow pages will be created
  	 * beyond this new batch of bucket pages.
  	 */
  	if (spare_ndx > metap->hashm_ovflpoint)
  	{
  		metap->hashm_spares[spare_ndx] = metap->hashm_spares[metap->hashm_ovflpoint];
  		metap->hashm_ovflpoint = spare_ndx;
  		metap_update_splitpoint = true;
  	}
  
  	MarkBufferDirty(metabuf);
  
  	/*
  	 * Copy bucket mapping info now; this saves re-accessing the meta page
  	 * inside _hash_splitbucket's inner loop.  Note that once we drop the
  	 * split lock, other splits could begin, so these values might be out of
  	 * date before _hash_splitbucket finishes.  That's okay, since all it
  	 * needs is to tell which of these two buckets to map hashkeys into.
  	 */
  	maxbucket = metap->hashm_maxbucket;
  	highmask = metap->hashm_highmask;
  	lowmask = metap->hashm_lowmask;
  
  	opage = BufferGetPage(buf_oblkno);
  	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
  
  	/*
  	 * Mark the old bucket to indicate that split is in progress.  (At
  	 * operation end, we will clear the split-in-progress flag.)  Also, for a
  	 * primary bucket page, hasho_prevblkno stores the number of buckets that
  	 * existed as of the last split, so we must update that value here.
  	 */
  	oopaque->hasho_flag |= LH_BUCKET_BEING_SPLIT;
  	oopaque->hasho_prevblkno = maxbucket;
  
  	MarkBufferDirty(buf_oblkno);
  
  	npage = BufferGetPage(buf_nblkno);
  
  	/*
  	 * initialize the new bucket's primary page and mark it to indicate that
  	 * split is in progress.
  	 */
  	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  	nopaque->hasho_prevblkno = maxbucket;
  	nopaque->hasho_nextblkno = InvalidBlockNumber;
  	nopaque->hasho_bucket = new_bucket;
  	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_BEING_POPULATED;
  	nopaque->hasho_page_id = HASHO_PAGE_ID;
  
  	MarkBufferDirty(buf_nblkno);
  
  	/* XLOG stuff */
  	if (RelationNeedsWAL(rel))
  	{
  		xl_hash_split_allocate_page xlrec;
  		XLogRecPtr	recptr;
  
  		xlrec.new_bucket = maxbucket;
  		xlrec.old_bucket_flag = oopaque->hasho_flag;
  		xlrec.new_bucket_flag = nopaque->hasho_flag;
  		xlrec.flags = 0;
  
  		XLogBeginInsert();
  
  		XLogRegisterBuffer(0, buf_oblkno, REGBUF_STANDARD);
  		XLogRegisterBuffer(1, buf_nblkno, REGBUF_WILL_INIT);
  		XLogRegisterBuffer(2, metabuf, REGBUF_STANDARD);
  
  		if (metap_update_masks)
  		{
  			xlrec.flags |= XLH_SPLIT_META_UPDATE_MASKS;
  			XLogRegisterBufData(2, (char *) &metap->hashm_lowmask, sizeof(uint32));
  			XLogRegisterBufData(2, (char *) &metap->hashm_highmask, sizeof(uint32));
  		}
  
  		if (metap_update_splitpoint)
  		{
  			xlrec.flags |= XLH_SPLIT_META_UPDATE_SPLITPOINT;
  			XLogRegisterBufData(2, (char *) &metap->hashm_ovflpoint,
  								sizeof(uint32));
  			XLogRegisterBufData(2,
  								(char *) &metap->hashm_spares[metap->hashm_ovflpoint],
  								sizeof(uint32));
  		}
  
  		XLogRegisterData((char *) &xlrec, SizeOfHashSplitAllocPage);
  
  		recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_ALLOCATE_PAGE);
  
  		PageSetLSN(BufferGetPage(buf_oblkno), recptr);
  		PageSetLSN(BufferGetPage(buf_nblkno), recptr);
  		PageSetLSN(BufferGetPage(metabuf), recptr);
  	}
  
  	END_CRIT_SECTION();
  
  	/* drop lock, but keep pin */
  	LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
  
  	/* Relocate records to the new bucket */
  	_hash_splitbucket(rel, metabuf,
  					  old_bucket, new_bucket,
  					  buf_oblkno, buf_nblkno, NULL,
  					  maxbucket, highmask, lowmask);
  
  	/* all done, now release the pins on primary buckets. */
  	_hash_dropbuf(rel, buf_oblkno);
  	_hash_dropbuf(rel, buf_nblkno);
  
  	return;
  
  	/* Here if decide not to split or fail to acquire old bucket lock */
  fail:
  
  	/* We didn't write the metapage, so just drop lock */
  	LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
  }
  
  
  /*
   * _hash_alloc_buckets -- allocate a new splitpoint's worth of bucket pages
   *
   * This does not need to initialize the new bucket pages; we'll do that as
   * each one is used by _hash_expandtable().  But we have to extend the logical
   * EOF to the end of the splitpoint; this keeps smgr's idea of the EOF in
   * sync with ours, so that we don't get complaints from smgr.
   *
   * We do this by writing a page of zeroes at the end of the splitpoint range.
   * We expect that the filesystem will ensure that the intervening pages read
   * as zeroes too.  On many filesystems this "hole" will not be allocated
   * immediately, which means that the index file may end up more fragmented
   * than if we forced it all to be allocated now; but since we don't scan
   * hash indexes sequentially anyway, that probably doesn't matter.
   *
   * XXX It's annoying that this code is executed with the metapage lock held.
   * We need to interlock against _hash_addovflpage() adding a new overflow page
   * concurrently, but it'd likely be better to use LockRelationForExtension
   * for the purpose.  OTOH, adding a splitpoint is a very infrequent operation,
   * so it may not be worth worrying about.
   *
   * Returns true if successful, or false if allocation failed due to
   * BlockNumber overflow.
   */
  static bool
  _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  {
  	BlockNumber lastblock;
  	PGAlignedBlock zerobuf;
  	Page		page;
  	HashPageOpaque ovflopaque;
  
  	lastblock = firstblock + nblocks - 1;
  
  	/*
  	 * Check for overflow in block number calculation; if so, we cannot extend
  	 * the index anymore.
  	 */
  	if (lastblock < firstblock || lastblock == InvalidBlockNumber)
  		return false;
  
  	page = (Page) zerobuf.data;
  
  	/*
  	 * Initialize the page.  Just zeroing the page won't work; see
  	 * _hash_freeovflpage for similar usage.  We take care to make the special
  	 * space valid for the benefit of tools such as pageinspect.
  	 */
  	_hash_pageinit(page, BLCKSZ);
  
  	ovflopaque = (HashPageOpaque) PageGetSpecialPointer(page);
  
  	ovflopaque->hasho_prevblkno = InvalidBlockNumber;
  	ovflopaque->hasho_nextblkno = InvalidBlockNumber;
  	ovflopaque->hasho_bucket = -1;
  	ovflopaque->hasho_flag = LH_UNUSED_PAGE;
  	ovflopaque->hasho_page_id = HASHO_PAGE_ID;
  
  	if (RelationNeedsWAL(rel))
  		log_newpage(&rel->rd_node,
  					MAIN_FORKNUM,
  					lastblock,
  					zerobuf.data,
  					true);
  
  	RelationOpenSmgr(rel);
  	PageSetChecksumInplace(page, lastblock);
  	smgrextend(rel->rd_smgr, MAIN_FORKNUM, lastblock, zerobuf.data, false);
  
  	return true;
  }
  
  
  /*
   * _hash_splitbucket -- split 'obucket' into 'obucket' and 'nbucket'
   *
   * This routine is used to partition the tuples between old and new bucket and
   * is used to finish the incomplete split operations.  To finish the previously
   * interrupted split operation, the caller needs to fill htab.  If htab is set,
   * then we skip the movement of tuples that exists in htab, otherwise NULL
   * value of htab indicates movement of all the tuples that belong to the new
   * bucket.
   *
   * We are splitting a bucket that consists of a base bucket page and zero
   * or more overflow (bucket chain) pages.  We must relocate tuples that
   * belong in the new bucket.
   *
   * The caller must hold cleanup locks on both buckets to ensure that
   * no one else is trying to access them (see README).
   *
   * The caller must hold a pin, but no lock, on the metapage buffer.
   * The buffer is returned in the same state.  (The metapage is only
   * touched if it becomes necessary to add or remove overflow pages.)
   *
   * Split needs to retain pin on primary bucket pages of both old and new
   * buckets till end of operation.  This is to prevent vacuum from starting
   * while a split is in progress.
   *
   * In addition, the caller must have created the new bucket's base page,
   * which is passed in buffer nbuf, pinned and write-locked.  The lock will be
   * released here and pin must be released by the caller.  (The API is set up
   * this way because we must do _hash_getnewbuf() before releasing the metapage
   * write lock.  So instead of passing the new bucket's start block number, we
   * pass an actual buffer.)
   */
  static void
  _hash_splitbucket(Relation rel,
  				  Buffer metabuf,
  				  Bucket obucket,
  				  Bucket nbucket,
  				  Buffer obuf,
  				  Buffer nbuf,
  				  HTAB *htab,
  				  uint32 maxbucket,
  				  uint32 highmask,
  				  uint32 lowmask)
  {
  	Buffer		bucket_obuf;
  	Buffer		bucket_nbuf;
  	Page		opage;
  	Page		npage;
  	HashPageOpaque oopaque;
  	HashPageOpaque nopaque;
  	OffsetNumber itup_offsets[MaxIndexTuplesPerPage];
  	IndexTuple	itups[MaxIndexTuplesPerPage];
  	Size		all_tups_size = 0;
  	int			i;
  	uint16		nitups = 0;
  
  	bucket_obuf = obuf;
  	opage = BufferGetPage(obuf);
  	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
  
  	bucket_nbuf = nbuf;
  	npage = BufferGetPage(nbuf);
  	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  
  	/* Copy the predicate locks from old bucket to new bucket. */
  	PredicateLockPageSplit(rel,
  						   BufferGetBlockNumber(bucket_obuf),
  						   BufferGetBlockNumber(bucket_nbuf));
  
  	/*
  	 * Partition the tuples in the old bucket between the old bucket and the
  	 * new bucket, advancing along the old bucket's overflow bucket chain and
  	 * adding overflow pages to the new bucket as needed.  Outer loop iterates
  	 * once per page in old bucket.
  	 */
  	for (;;)
  	{
  		BlockNumber oblkno;
  		OffsetNumber ooffnum;
  		OffsetNumber omaxoffnum;
  
  		/* Scan each tuple in old page */
  		omaxoffnum = PageGetMaxOffsetNumber(opage);
  		for (ooffnum = FirstOffsetNumber;
  			 ooffnum <= omaxoffnum;
  			 ooffnum = OffsetNumberNext(ooffnum))
  		{
  			IndexTuple	itup;
  			Size		itemsz;
  			Bucket		bucket;
  			bool		found = false;
  
  			/* skip dead tuples */
  			if (ItemIdIsDead(PageGetItemId(opage, ooffnum)))
  				continue;
  
  			/*
  			 * Before inserting a tuple, probe the hash table containing TIDs
  			 * of tuples belonging to new bucket, if we find a match, then
  			 * skip that tuple, else fetch the item's hash key (conveniently
  			 * stored in the item) and determine which bucket it now belongs
  			 * in.
  			 */
  			itup = (IndexTuple) PageGetItem(opage,
  											PageGetItemId(opage, ooffnum));
  
  			if (htab)
  				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
  
  			if (found)
  				continue;
  
  			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
  										  maxbucket, highmask, lowmask);
  
  			if (bucket == nbucket)
  			{
  				IndexTuple	new_itup;
  
  				/*
  				 * make a copy of index tuple as we have to scribble on it.
  				 */
  				new_itup = CopyIndexTuple(itup);
  
  				/*
  				 * mark the index tuple as moved by split, such tuples are
  				 * skipped by scan if there is split in progress for a bucket.
  				 */
  				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
  
  				/*
  				 * insert the tuple into the new bucket.  if it doesn't fit on
  				 * the current page in the new bucket, we must allocate a new
  				 * overflow page and place the tuple on that page instead.
  				 */
  				itemsz = IndexTupleSize(new_itup);
  				itemsz = MAXALIGN(itemsz);
  
  				if (PageGetFreeSpaceForMultipleTuples(npage, nitups + 1) < (all_tups_size + itemsz))
  				{
  					/*
  					 * Change the shared buffer state in critical section,
  					 * otherwise any error could make it unrecoverable.
  					 */
  					START_CRIT_SECTION();
  
  					_hash_pgaddmultitup(rel, nbuf, itups, itup_offsets, nitups);
  					MarkBufferDirty(nbuf);
  					/* log the split operation before releasing the lock */
  					log_split_page(rel, nbuf);
  
  					END_CRIT_SECTION();
  
  					/* drop lock, but keep pin */
  					LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
  
  					/* be tidy */
  					for (i = 0; i < nitups; i++)
  						pfree(itups[i]);
  					nitups = 0;
  					all_tups_size = 0;
  
  					/* chain to a new overflow page */
  					nbuf = _hash_addovflpage(rel, metabuf, nbuf, (nbuf == bucket_nbuf) ? true : false);
  					npage = BufferGetPage(nbuf);
  					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  				}
  
  				itups[nitups++] = new_itup;
  				all_tups_size += itemsz;
  			}
  			else
  			{
  				/*
  				 * the tuple stays on this page, so nothing to do.
  				 */
  				Assert(bucket == obucket);
  			}
  		}
  
  		oblkno = oopaque->hasho_nextblkno;
  
  		/* retain the pin on the old primary bucket */
  		if (obuf == bucket_obuf)
  			LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
  		else
  			_hash_relbuf(rel, obuf);
  
  		/* Exit loop if no more overflow pages in old bucket */
  		if (!BlockNumberIsValid(oblkno))
  		{
  			/*
  			 * Change the shared buffer state in critical section, otherwise
  			 * any error could make it unrecoverable.
  			 */
  			START_CRIT_SECTION();
  
  			_hash_pgaddmultitup(rel, nbuf, itups, itup_offsets, nitups);
  			MarkBufferDirty(nbuf);
  			/* log the split operation before releasing the lock */
  			log_split_page(rel, nbuf);
  
  			END_CRIT_SECTION();
  
  			if (nbuf == bucket_nbuf)
  				LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
  			else
  				_hash_relbuf(rel, nbuf);
  
  			/* be tidy */
  			for (i = 0; i < nitups; i++)
  				pfree(itups[i]);
  			break;
  		}
  
  		/* Else, advance to next old page */
  		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
  		opage = BufferGetPage(obuf);
  		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
  	}
  
  	/*
  	 * We're at the end of the old bucket chain, so we're done partitioning
  	 * the tuples.  Mark the old and new buckets to indicate split is
  	 * finished.
  	 *
  	 * To avoid deadlocks due to locking order of buckets, first lock the old
  	 * bucket and then the new bucket.
  	 */
  	LockBuffer(bucket_obuf, BUFFER_LOCK_EXCLUSIVE);
  	opage = BufferGetPage(bucket_obuf);
  	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
  
  	LockBuffer(bucket_nbuf, BUFFER_LOCK_EXCLUSIVE);
  	npage = BufferGetPage(bucket_nbuf);
  	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  
  	START_CRIT_SECTION();
  
  	oopaque->hasho_flag &= ~LH_BUCKET_BEING_SPLIT;
  	nopaque->hasho_flag &= ~LH_BUCKET_BEING_POPULATED;
  
  	/*
  	 * After the split is finished, mark the old bucket to indicate that it
  	 * contains deletable tuples.  We will clear split-cleanup flag after
  	 * deleting such tuples either at the end of split or at the next split
  	 * from old bucket or at the time of vacuum.
  	 */
  	oopaque->hasho_flag |= LH_BUCKET_NEEDS_SPLIT_CLEANUP;
  
  	/*
  	 * now write the buffers, here we don't release the locks as caller is
  	 * responsible to release locks.
  	 */
  	MarkBufferDirty(bucket_obuf);
  	MarkBufferDirty(bucket_nbuf);
  
  	if (RelationNeedsWAL(rel))
  	{
  		XLogRecPtr	recptr;
  		xl_hash_split_complete xlrec;
  
  		xlrec.old_bucket_flag = oopaque->hasho_flag;
  		xlrec.new_bucket_flag = nopaque->hasho_flag;
  
  		XLogBeginInsert();
  
  		XLogRegisterData((char *) &xlrec, SizeOfHashSplitComplete);
  
  		XLogRegisterBuffer(0, bucket_obuf, REGBUF_STANDARD);
  		XLogRegisterBuffer(1, bucket_nbuf, REGBUF_STANDARD);
  
  		recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_COMPLETE);
  
  		PageSetLSN(BufferGetPage(bucket_obuf), recptr);
  		PageSetLSN(BufferGetPage(bucket_nbuf), recptr);
  	}
  
  	END_CRIT_SECTION();
  
  	/*
  	 * If possible, clean up the old bucket.  We might not be able to do this
  	 * if someone else has a pin on it, but if not then we can go ahead.  This
  	 * isn't absolutely necessary, but it reduces bloat; if we don't do it
  	 * now, VACUUM will do it eventually, but maybe not until new overflow
  	 * pages have been allocated.  Note that there's no need to clean up the
  	 * new bucket.
  	 */
  	if (IsBufferCleanupOK(bucket_obuf))
  	{
  		LockBuffer(bucket_nbuf, BUFFER_LOCK_UNLOCK);
  		hashbucketcleanup(rel, obucket, bucket_obuf,
  						  BufferGetBlockNumber(bucket_obuf), NULL,
  						  maxbucket, highmask, lowmask, NULL, NULL, true,
  						  NULL, NULL);
  	}
  	else
  	{
  		LockBuffer(bucket_nbuf, BUFFER_LOCK_UNLOCK);
  		LockBuffer(bucket_obuf, BUFFER_LOCK_UNLOCK);
  	}
  }
  
  /*
   *	_hash_finish_split() -- Finish the previously interrupted split operation
   *
   * To complete the split operation, we form the hash table of TIDs in new
   * bucket which is then used by split operation to skip tuples that are
   * already moved before the split operation was previously interrupted.
   *
   * The caller must hold a pin, but no lock, on the metapage and old bucket's
   * primary page buffer.  The buffers are returned in the same state.  (The
   * metapage is only touched if it becomes necessary to add or remove overflow
   * pages.)
   */
  void
  _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Bucket obucket,
  				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
  {
  	HASHCTL		hash_ctl;
  	HTAB	   *tidhtab;
  	Buffer		bucket_nbuf = InvalidBuffer;
  	Buffer		nbuf;
  	Page		npage;
  	BlockNumber nblkno;
  	BlockNumber bucket_nblkno;
  	HashPageOpaque npageopaque;
  	Bucket		nbucket;
  	bool		found;
  
  	/* Initialize hash tables used to track TIDs */
  	memset(&hash_ctl, 0, sizeof(hash_ctl));
  	hash_ctl.keysize = sizeof(ItemPointerData);
  	hash_ctl.entrysize = sizeof(ItemPointerData);
  	hash_ctl.hcxt = CurrentMemoryContext;
  
  	tidhtab =
  		hash_create("bucket ctids",
  					256,		/* arbitrary initial size */
  					&hash_ctl,
  					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  
  	bucket_nblkno = nblkno = _hash_get_newblock_from_oldbucket(rel, obucket);
  
  	/*
  	 * Scan the new bucket and build hash table of TIDs
  	 */
  	for (;;)
  	{
  		OffsetNumber noffnum;
  		OffsetNumber nmaxoffnum;
  
  		nbuf = _hash_getbuf(rel, nblkno, HASH_READ,
  							LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
  
  		/* remember the primary bucket buffer to acquire cleanup lock on it. */
  		if (nblkno == bucket_nblkno)
  			bucket_nbuf = nbuf;
  
  		npage = BufferGetPage(nbuf);
  		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  
  		/* Scan each tuple in new page */
  		nmaxoffnum = PageGetMaxOffsetNumber(npage);
  		for (noffnum = FirstOffsetNumber;
  			 noffnum <= nmaxoffnum;
  			 noffnum = OffsetNumberNext(noffnum))
  		{
  			IndexTuple	itup;
  
  			/* Fetch the item's TID and insert it in hash table. */
  			itup = (IndexTuple) PageGetItem(npage,
  											PageGetItemId(npage, noffnum));
  
  			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
  
  			Assert(!found);
  		}
  
  		nblkno = npageopaque->hasho_nextblkno;
  
  		/*
  		 * release our write lock without modifying buffer and ensure to
  		 * retain the pin on primary bucket.
  		 */
  		if (nbuf == bucket_nbuf)
  			LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
  		else
  			_hash_relbuf(rel, nbuf);
  
  		/* Exit loop if no more overflow pages in new bucket */
  		if (!BlockNumberIsValid(nblkno))
  			break;
  	}
  
  	/*
  	 * Conditionally get the cleanup lock on old and new buckets to perform
  	 * the split operation.  If we don't get the cleanup locks, silently give
  	 * up and next insertion on old bucket will try again to complete the
  	 * split.
  	 */
  	if (!ConditionalLockBufferForCleanup(obuf))
  	{
  		hash_destroy(tidhtab);
  		return;
  	}
  	if (!ConditionalLockBufferForCleanup(bucket_nbuf))
  	{
  		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
  		hash_destroy(tidhtab);
  		return;
  	}
  
  	npage = BufferGetPage(bucket_nbuf);
  	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
  	nbucket = npageopaque->hasho_bucket;
  
  	_hash_splitbucket(rel, metabuf, obucket,
  					  nbucket, obuf, bucket_nbuf, tidhtab,
  					  maxbucket, highmask, lowmask);
  
  	_hash_dropbuf(rel, bucket_nbuf);
  	hash_destroy(tidhtab);
  }
  
  /*
   *	log_split_page() -- Log the split operation
   *
   *	We log the split operation when the new page in new bucket gets full,
   *	so we log the entire page.
   *
   *	'buf' must be locked by the caller which is also responsible for unlocking
   *	it.
   */
  static void
  log_split_page(Relation rel, Buffer buf)
  {
  	if (RelationNeedsWAL(rel))
  	{
  		XLogRecPtr	recptr;
  
  		XLogBeginInsert();
  
  		XLogRegisterBuffer(0, buf, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
  
  		recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_PAGE);
  
  		PageSetLSN(BufferGetPage(buf), recptr);
  	}
  }
  
  /*
   *	_hash_getcachedmetap() -- Returns cached metapage data.
   *
   *	If metabuf is not InvalidBuffer, caller must hold a pin, but no lock, on
   *	the metapage.  If not set, we'll set it before returning if we have to
   *	refresh the cache, and return with a pin but no lock on it; caller is
   *	responsible for releasing the pin.
   *
   *	We refresh the cache if it's not initialized yet or force_refresh is true.
   */
  HashMetaPage
  _hash_getcachedmetap(Relation rel, Buffer *metabuf, bool force_refresh)
  {
  	Page		page;
  
  	Assert(metabuf);
  	if (force_refresh || rel->rd_amcache == NULL)
  	{
  		char	   *cache = NULL;
  
  		/*
  		 * It's important that we don't set rd_amcache to an invalid value.
  		 * Either MemoryContextAlloc or _hash_getbuf could fail, so don't
  		 * install a pointer to the newly-allocated storage in the actual
  		 * relcache entry until both have succeeeded.
  		 */
  		if (rel->rd_amcache == NULL)
  			cache = MemoryContextAlloc(rel->rd_indexcxt,
  									   sizeof(HashMetaPageData));
  
  		/* Read the metapage. */
  		if (BufferIsValid(*metabuf))
  			LockBuffer(*metabuf, BUFFER_LOCK_SHARE);
  		else
  			*metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ,
  									LH_META_PAGE);
  		page = BufferGetPage(*metabuf);
  
  		/* Populate the cache. */
  		if (rel->rd_amcache == NULL)
  			rel->rd_amcache = cache;
  		memcpy(rel->rd_amcache, HashPageGetMeta(page),
  			   sizeof(HashMetaPageData));
  
  		/* Release metapage lock, but keep the pin. */
  		LockBuffer(*metabuf, BUFFER_LOCK_UNLOCK);
  	}
  
  	return (HashMetaPage) rel->rd_amcache;
  }
  
  /*
   *	_hash_getbucketbuf_from_hashkey() -- Get the bucket's buffer for the given
   *										 hashkey.
   *
   *	Bucket pages do not move or get removed once they are allocated. This give
   *	us an opportunity to use the previously saved metapage contents to reach
   *	the target bucket buffer, instead of reading from the metapage every time.
   *	This saves one buffer access every time we want to reach the target bucket
   *	buffer, which is very helpful savings in bufmgr traffic and contention.
   *
   *	The access type parameter (HASH_READ or HASH_WRITE) indicates whether the
   *	bucket buffer has to be locked for reading or writing.
   *
   *	The out parameter cachedmetap is set with metapage contents used for
   *	hashkey to bucket buffer mapping. Some callers need this info to reach the
   *	old bucket in case of bucket split, see _hash_doinsert().
   */
  Buffer
  _hash_getbucketbuf_from_hashkey(Relation rel, uint32 hashkey, int access,
  								HashMetaPage *cachedmetap)
  {
  	HashMetaPage metap;
  	Buffer		buf;
  	Buffer		metabuf = InvalidBuffer;
  	Page		page;
  	Bucket		bucket;
  	BlockNumber blkno;
  	HashPageOpaque opaque;
  
  	/* We read from target bucket buffer, hence locking is must. */
  	Assert(access == HASH_READ || access == HASH_WRITE);
  
  	metap = _hash_getcachedmetap(rel, &metabuf, false);
  	Assert(metap != NULL);
  
  	/*
  	 * Loop until we get a lock on the correct target bucket.
  	 */
  	for (;;)
  	{
  		/*
  		 * Compute the target bucket number, and convert to block number.
  		 */
  		bucket = _hash_hashkey2bucket(hashkey,
  									  metap->hashm_maxbucket,
  									  metap->hashm_highmask,
  									  metap->hashm_lowmask);
  
  		blkno = BUCKET_TO_BLKNO(metap, bucket);
  
  		/* Fetch the primary bucket page for the bucket */
  		buf = _hash_getbuf(rel, blkno, access, LH_BUCKET_PAGE);
  		page = BufferGetPage(buf);
  		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
  		Assert(opaque->hasho_bucket == bucket);
  		Assert(opaque->hasho_prevblkno != InvalidBlockNumber);
  
  		/*
  		 * If this bucket hasn't been split, we're done.
  		 */
  		if (opaque->hasho_prevblkno <= metap->hashm_maxbucket)
  			break;
  
  		/* Drop lock on this buffer, update cached metapage, and retry. */
  		_hash_relbuf(rel, buf);
  		metap = _hash_getcachedmetap(rel, &metabuf, true);
  		Assert(metap != NULL);
  	}
  
  	if (BufferIsValid(metabuf))
  		_hash_dropbuf(rel, metabuf);
  
  	if (cachedmetap)
  		*cachedmetap = metap;
  
  	return buf;
  }
  