text: /*
  **
  ** The author disclaims copyright to this source code.  In place of
  ** a legal notice, here is a blessing:
  **
  **    May you do good and not evil.
  **    May you find forgiveness for yourself and forgive others.
  **    May you share freely, never taking more than you give.
  **
  *************************************************************************
  ** This file contains code used by the compiler to add foreign key
  ** support to compiled SQL statements.
  */
  #include "sqliteInt.h"
  
  #ifndef SQLITE_OMIT_FOREIGN_KEY
  #ifndef SQLITE_OMIT_TRIGGER
  
  /*
  ** Deferred and Immediate FKs
  ** --------------------------
  **
  ** Foreign keys in SQLite come in two flavours: deferred and immediate.
  ** If an immediate foreign key constraint is violated,
  ** SQLITE_CONSTRAINT_FOREIGNKEY is returned and the current
  ** statement transaction rolled back. If a 
  ** deferred foreign key constraint is violated, no action is taken 
  ** immediately. However if the application attempts to commit the 
  ** transaction before fixing the constraint violation, the attempt fails.
  **
  ** Deferred constraints are implemented using a simple counter associated
  ** with the database handle. The counter is set to zero each time a 
  ** database transaction is opened. Each time a statement is executed 
  ** that causes a foreign key violation, the counter is incremented. Each
  ** time a statement is executed that removes an existing violation from
  ** the database, the counter is decremented. When the transaction is
  ** committed, the commit fails if the current value of the counter is
  ** greater than zero. This scheme has two big drawbacks:
  **
  **   * When a commit fails due to a deferred foreign key constraint, 
  **     there is no way to tell which foreign constraint is not satisfied,
  **     or which row it is not satisfied for.
  **
  **   * If the database contains foreign key violations when the 
  **     transaction is opened, this may cause the mechanism to malfunction.
  **
  ** Despite these problems, this approach is adopted as it seems simpler
  ** than the alternatives.
  **
  ** INSERT operations:
  **
  **   I.1) For each FK for which the table is the child table, search
  **        the parent table for a match. If none is found increment the
  **        constraint counter.
  **
  **   I.2) For each FK for which the table is the parent table, 
  **        search the child table for rows that correspond to the new
  **        row in the parent table. Decrement the counter for each row
  **        found (as the constraint is now satisfied).
  **
  ** DELETE operations:
  **
  **   D.1) For each FK for which the table is the child table, 
  **        search the parent table for a row that corresponds to the 
  **        deleted row in the child table. If such a row is not found, 
  **        decrement the counter.
  **
  **   D.2) For each FK for which the table is the parent table, search 
  **        the child table for rows that correspond to the deleted row 
  **        in the parent table. For each found increment the counter.
  **
  ** UPDATE operations:
  **
  **   An UPDATE command requires that all 4 steps above are taken, but only
  **   for FK constraints for which the affected columns are actually 
  **   modified (values must be compared at runtime).
  **
  ** Note that I.1 and D.1 are very similar operations, as are I.2 and D.2.
  ** This simplifies the implementation a bit.
  **
  ** For the purposes of immediate FK constraints, the OR REPLACE conflict
  ** resolution is considered to delete rows before the new row is inserted.
  ** If a delete caused by OR REPLACE violates an FK constraint, an exception
  ** is thrown, even if the FK constraint would be satisfied after the new 
  ** row is inserted.
  **
  ** Immediate constraints are usually handled similarly. The only difference 
  ** is that the counter used is stored as part of each individual statement
  ** object (struct Vdbe). If, after the statement has run, its immediate
  ** constraint counter is greater than zero,
  ** it returns SQLITE_CONSTRAINT_FOREIGNKEY
  ** and the statement transaction is rolled back. An exception is an INSERT
  ** statement that inserts a single row only (no triggers). In this case,
  ** instead of using a counter, an exception is thrown immediately if the
  ** INSERT violates a foreign key constraint. This is necessary as such
  ** an INSERT does not open a statement transaction.
  **
  ** TODO: How should dropping a table be handled? How should renaming a 
  ** table be handled?
  **
  **
  ** Query API Notes
  ** ---------------
  **
  ** Before coding an UPDATE or DELETE row operation, the code-generator
  ** for those two operations needs to know whether or not the operation
  ** requires any FK processing and, if so, which columns of the original
  ** row are required by the FK processing VDBE code (i.e. if FKs were
  ** implemented using triggers, which of the old.* columns would be 
  ** accessed). No information is required by the code-generator before
  ** coding an INSERT operation. The functions used by the UPDATE/DELETE
  ** generation code to query for this information are:
  **
  **   sqlite3FkRequired() - Test to see if FK processing is required.
  **   sqlite3FkOldmask()  - Query for the set of required old.* columns.
  **
  **
  ** Externally accessible module functions
  ** --------------------------------------
  **
  **   sqlite3FkCheck()    - Check for foreign key violations.
  **   sqlite3FkActions()  - Code triggers for ON UPDATE/ON DELETE actions.
  **   sqlite3FkDelete()   - Delete an FKey structure.
  */
  
  /*
  ** VDBE Calling Convention
  ** -----------------------
  **
  ** Example:
  **
  **   For the following INSERT statement:
  **
  **     CREATE TABLE t1(a, b INTEGER PRIMARY KEY, c);
  **     INSERT INTO t1 VALUES(1, 2, 3.1);
  **
  **   Register (x):        2    (type integer)
  **   Register (x+1):      1    (type integer)
  **   Register (x+2):      NULL (type NULL)
  **   Register (x+3):      3.1  (type real)
  */
  
  /*
  ** A foreign key constraint requires that the key columns in the parent
  ** table are collectively subject to a UNIQUE or PRIMARY KEY constraint.
  ** Given that pParent is the parent table for foreign key constraint pFKey, 
  ** search the schema for a unique index on the parent key columns. 
  **
  ** If successful, zero is returned. If the parent key is an INTEGER PRIMARY 
  ** KEY column, then output variable *ppIdx is set to NULL. Otherwise, *ppIdx 
  ** is set to point to the unique index. 
  ** 
  ** If the parent key consists of a single column (the foreign key constraint
  ** is not a composite foreign key), output variable *paiCol is set to NULL.
  ** Otherwise, it is set to point to an allocated array of size N, where
  ** N is the number of columns in the parent key. The first element of the
  ** array is the index of the child table column that is mapped by the FK
  ** constraint to the parent table column stored in the left-most column
  ** of index *ppIdx. The second element of the array is the index of the
  ** child table column that corresponds to the second left-most column of
  ** *ppIdx, and so on.
  **
  ** If the required index cannot be found, either because:
  **
  **   1) The named parent key columns do not exist, or
  **
  **   2) The named parent key columns do exist, but are not subject to a
  **      UNIQUE or PRIMARY KEY constraint, or
  **
  **   3) No parent key columns were provided explicitly as part of the
  **      foreign key definition, and the parent table does not have a
  **      PRIMARY KEY, or
  **
  **   4) No parent key columns were provided explicitly as part of the
  **      foreign key definition, and the PRIMARY KEY of the parent table 
  **      consists of a different number of columns to the child key in 
  **      the child table.
  **
  ** then non-zero is returned, and a "foreign key mismatch" error loaded
  ** into pParse. If an OOM error occurs, non-zero is returned and the
  ** pParse->db->mallocFailed flag is set.
  */
  int sqlite3FkLocateIndex(
    Parse *pParse,                  /* Parse context to store any error in */
    Table *pParent,                 /* Parent table of FK constraint pFKey */
    FKey *pFKey,                    /* Foreign key to find index for */
    Index **ppIdx,                  /* OUT: Unique index on parent table */
    int **paiCol                    /* OUT: Map of index columns in pFKey */
  ){
    Index *pIdx = 0;                    /* Value to return via *ppIdx */
    int *aiCol = 0;                     /* Value to return via *paiCol */
    int nCol = pFKey->nCol;             /* Number of columns in parent key */
    char *zKey = pFKey->aCol[0].zCol;   /* Name of left-most parent key column */
  
    /* The caller is responsible for zeroing output parameters. */
    assert( ppIdx && *ppIdx==0 );
    assert( !paiCol || *paiCol==0 );
    assert( pParse );
  
    /* If this is a non-composite (single column) foreign key, check if it 
    ** maps to the INTEGER PRIMARY KEY of table pParent. If so, leave *ppIdx 
    ** and *paiCol set to zero and return early. 
    **
    ** Otherwise, for a composite foreign key (more than one column), allocate
    ** space for the aiCol array (returned via output parameter *paiCol).
    ** Non-composite foreign keys do not require the aiCol array.
    */
    if( nCol==1 ){
      /* The FK maps to the IPK if any of the following are true:
      **
      **   1) There is an INTEGER PRIMARY KEY column and the FK is implicitly 
      **      mapped to the primary key of table pParent, or
      **   2) The FK is explicitly mapped to a column declared as INTEGER
      **      PRIMARY KEY.
      */
      if( pParent->iPKey>=0 ){
        if( !zKey ) return 0;
        if( !sqlite3StrICmp(pParent->aCol[pParent->iPKey].zName, zKey) ) return 0;
      }
    }else if( paiCol ){
      assert( nCol>1 );
      aiCol = (int *)sqlite3DbMallocRawNN(pParse->db, nCol*sizeof(int));
      if( !aiCol ) return 1;
      *paiCol = aiCol;
    }
  
    for(pIdx=pParent->pIndex; pIdx; pIdx=pIdx->pNext){
      if( pIdx->nKeyCol==nCol && IsUniqueIndex(pIdx) ){ 
        /* pIdx is a UNIQUE index (or a PRIMARY KEY) and has the right number
        ** of columns. If each indexed column corresponds to a foreign key
        ** column of pFKey, then this index is a winner.  */
  
        if( zKey==0 ){
          /* If zKey is NULL, then this foreign key is implicitly mapped to 
          ** the PRIMARY KEY of table pParent. The PRIMARY KEY index may be 
          ** identified by the test.  */
          if( IsPrimaryKeyIndex(pIdx) ){
            if( aiCol ){
              int i;
              for(i=0; i<nCol; i++) aiCol[i] = pFKey->aCol[i].iFrom;
            }
            break;
          }
        }else{
          /* If zKey is non-NULL, then this foreign key was declared to
          ** map to an explicit list of columns in table pParent. Check if this
          ** index matches those columns. Also, check that the index uses
          ** the default collation sequences for each column. */
          int i, j;
          for(i=0; i<nCol; i++){
            i16 iCol = pIdx->aiColumn[i];     /* Index of column in parent tbl */
            const char *zDfltColl;            /* Def. collation for column */
            char *zIdxCol;                    /* Name of indexed column */
  
            if( iCol<0 ) break; /* No foreign keys against expression indexes */
  
            /* If the index uses a collation sequence that is different from
            ** the default collation sequence for the column, this index is
            ** unusable. Bail out early in this case.  */
            zDfltColl = pParent->aCol[iCol].zColl;
            if( !zDfltColl ) zDfltColl = sqlite3StrBINARY;
            if( sqlite3StrICmp(pIdx->azColl[i], zDfltColl) ) break;
  
            zIdxCol = pParent->aCol[iCol].zName;
            for(j=0; j<nCol; j++){
              if( sqlite3StrICmp(pFKey->aCol[j].zCol, zIdxCol)==0 ){
                if( aiCol ) aiCol[i] = pFKey->aCol[j].iFrom;
                break;
              }
            }
            if( j==nCol ) break;
          }
          if( i==nCol ) break;      /* pIdx is usable */
        }
      }
    }
  
    if( !pIdx ){
      if( !pParse->disableTriggers ){
        sqlite3ErrorMsg(pParse,
             "foreign key mismatch - \"%w\" referencing \"%w\"",
             pFKey->pFrom->zName, pFKey->zTo);
      }
      sqlite3DbFree(pParse->db, aiCol);
      return 1;
    }
  
    *ppIdx = pIdx;
    return 0;
  }
  
  /*
  ** This function is called when a row is inserted into or deleted from the 
  ** child table of foreign key constraint pFKey. If an SQL UPDATE is executed 
  ** on the child table of pFKey, this function is invoked twice for each row
  ** affected - once to "delete" the old row, and then again to "insert" the
  ** new row.
  **
  ** Each time it is called, this function generates VDBE code to locate the
  ** row in the parent table that corresponds to the row being inserted into 
  ** or deleted from the child table. If the parent row can be found, no 
  ** special action is taken. Otherwise, if the parent row can *not* be
  ** found in the parent table:
  **
  **   Operation | FK type   | Action taken
  **   --------------------------------------------------------------------------
  **   INSERT      immediate   Increment the "immediate constraint counter".
  **
  **   DELETE      immediate   Decrement the "immediate constraint counter".
  **
  **   INSERT      deferred    Increment the "deferred constraint counter".
  **
  **   DELETE      deferred    Decrement the "deferred constraint counter".
  **
  ** These operations are identified in the comment at the top of this file 
  ** (fkey.c) as "I.1" and "D.1".
  */
  static void fkLookupParent(
    Parse *pParse,        /* Parse context */
    int iDb,              /* Index of database housing pTab */
    Table *pTab,          /* Parent table of FK pFKey */
    Index *pIdx,          /* Unique index on parent key columns in pTab */
    FKey *pFKey,          /* Foreign key constraint */
    int *aiCol,           /* Map from parent key columns to child table columns */
    int regData,          /* Address of array containing child table row */
    int nIncr,            /* Increment constraint counter by this */
    int isIgnore          /* If true, pretend pTab contains all NULL values */
  ){
    int i;                                    /* Iterator variable */
    Vdbe *v = sqlite3GetVdbe(pParse);         /* Vdbe to add code to */
    int iCur = pParse->nTab - 1;              /* Cursor number to use */
    int iOk = sqlite3VdbeMakeLabel(v);        /* jump here if parent key found */
  
    /* If nIncr is less than zero, then check at runtime if there are any
    ** outstanding constraints to resolve. If there are not, there is no need
    ** to check if deleting this row resolves any outstanding violations.
    **
    ** Check if any of the key columns in the child table row are NULL. If 
    ** any are, then the constraint is considered satisfied. No need to 
    ** search for a matching row in the parent table.  */
    if( nIncr<0 ){
      sqlite3VdbeAddOp2(v, OP_FkIfZero, pFKey->isDeferred, iOk);
      VdbeCoverage(v);
    }
    for(i=0; i<pFKey->nCol; i++){
      int iReg = aiCol[i] + regData + 1;
      sqlite3VdbeAddOp2(v, OP_IsNull, iReg, iOk); VdbeCoverage(v);
    }
  
    if( isIgnore==0 ){
      if( pIdx==0 ){
        /* If pIdx is NULL, then the parent key is the INTEGER PRIMARY KEY
        ** column of the parent table (table pTab).  */
        int iMustBeInt;               /* Address of MustBeInt instruction */
        int regTemp = sqlite3GetTempReg(pParse);
    
        /* Invoke MustBeInt to coerce the child key value to an integer (i.e. 
        ** apply the affinity of the parent key). If this fails, then there
        ** is no matching parent key. Before using MustBeInt, make a copy of
        ** the value. Otherwise, the value inserted into the child key column
        ** will have INTEGER affinity applied to it, which may not be correct.  */
        sqlite3VdbeAddOp2(v, OP_SCopy, aiCol[0]+1+regData, regTemp);
        iMustBeInt = sqlite3VdbeAddOp2(v, OP_MustBeInt, regTemp, 0);
        VdbeCoverage(v);
    
        /* If the parent table is the same as the child table, and we are about
        ** to increment the constraint-counter (i.e. this is an INSERT operation),
        ** then check if the row being inserted matches itself. If so, do not
        ** increment the constraint-counter.  */
        if( pTab==pFKey->pFrom && nIncr==1 ){
          sqlite3VdbeAddOp3(v, OP_Eq, regData, iOk, regTemp); VdbeCoverage(v);
          sqlite3VdbeChangeP5(v, SQLITE_NOTNULL);
        }
    
        sqlite3OpenTable(pParse, iCur, iDb, pTab, OP_OpenRead);
        sqlite3VdbeAddOp3(v, OP_NotExists, iCur, 0, regTemp); VdbeCoverage(v);
        sqlite3VdbeGoto(v, iOk);
        sqlite3VdbeJumpHere(v, sqlite3VdbeCurrentAddr(v)-2);
        sqlite3VdbeJumpHere(v, iMustBeInt);
        sqlite3ReleaseTempReg(pParse, regTemp);
      }else{
        int nCol = pFKey->nCol;
        int regTemp = sqlite3GetTempRange(pParse, nCol);
        int regRec = sqlite3GetTempReg(pParse);
    
        sqlite3VdbeAddOp3(v, OP_OpenRead, iCur, pIdx->tnum, iDb);
        sqlite3VdbeSetP4KeyInfo(pParse, pIdx);
        for(i=0; i<nCol; i++){
          sqlite3VdbeAddOp2(v, OP_Copy, aiCol[i]+1+regData, regTemp+i);
        }
    
        /* If the parent table is the same as the child table, and we are about
        ** to increment the constraint-counter (i.e. this is an INSERT operation),
        ** then check if the row being inserted matches itself. If so, do not
        ** increment the constraint-counter. 
        **
        ** If any of the parent-key values are NULL, then the row cannot match 
        ** itself. So set JUMPIFNULL to make sure we do the OP_Found if any
        ** of the parent-key values are NULL (at this point it is known that
        ** none of the child key values are).
        */
        if( pTab==pFKey->pFrom && nIncr==1 ){
          int iJump = sqlite3VdbeCurrentAddr(v) + nCol + 1;
          for(i=0; i<nCol; i++){
            int iChild = aiCol[i]+1+regData;
            int iParent = pIdx->aiColumn[i]+1+regData;
            assert( pIdx->aiColumn[i]>=0 );
            assert( aiCol[i]!=pTab->iPKey );
            if( pIdx->aiColumn[i]==pTab->iPKey ){
              /* The parent key is a composite key that includes the IPK column */
              iParent = regData;
            }
            sqlite3VdbeAddOp3(v, OP_Ne, iChild, iJump, iParent); VdbeCoverage(v);
            sqlite3VdbeChangeP5(v, SQLITE_JUMPIFNULL);
          }
          sqlite3VdbeGoto(v, iOk);
        }
    
        sqlite3VdbeAddOp4(v, OP_MakeRecord, regTemp, nCol, regRec,
                          sqlite3IndexAffinityStr(pParse->db,pIdx), nCol);
        sqlite3VdbeAddOp4Int(v, OP_Found, iCur, iOk, regRec, 0); VdbeCoverage(v);
    
        sqlite3ReleaseTempReg(pParse, regRec);
        sqlite3ReleaseTempRange(pParse, regTemp, nCol);
      }
    }
  
    if( !pFKey->isDeferred && !(pParse->db->flags & SQLITE_DeferFKs)
     && !pParse->pToplevel 
     && !pParse->isMultiWrite 
    ){
      /* Special case: If this is an INSERT statement that will insert exactly
      ** one row into the table, raise a constraint immediately instead of
      ** incrementing a counter. This is necessary as the VM code is being
      ** generated for will not open a statement transaction.  */
      assert( nIncr==1 );
      sqlite3HaltConstraint(pParse, SQLITE_CONSTRAINT_FOREIGNKEY,
          OE_Abort, 0, P4_STATIC, P5_ConstraintFK);
    }else{
      if( nIncr>0 && pFKey->isDeferred==0 ){
        sqlite3MayAbort(pParse);
      }
      sqlite3VdbeAddOp2(v, OP_FkCounter, pFKey->isDeferred, nIncr);
    }
  
    sqlite3VdbeResolveLabel(v, iOk);
    sqlite3VdbeAddOp1(v, OP_Close, iCur);
  }
  
  
  /*
  ** Return an Expr object that refers to a memory register corresponding
  ** to column iCol of table pTab.
  **
  ** regBase is the first of an array of register that contains the data
  ** for pTab.  regBase itself holds the rowid.  regBase+1 holds the first
  ** column.  regBase+2 holds the second column, and so forth.
  */
  static Expr *exprTableRegister(
    Parse *pParse,     /* Parsing and code generating context */
    Table *pTab,       /* The table whose content is at r[regBase]... */
    int regBase,       /* Contents of table pTab */
    i16 iCol           /* Which column of pTab is desired */
  ){
    Expr *pExpr;
    Column *pCol;
    const char *zColl;
    sqlite3 *db = pParse->db;
  
    pExpr = sqlite3Expr(db, TK_REGISTER, 0);
    if( pExpr ){
      if( iCol>=0 && iCol!=pTab->iPKey ){
        pCol = &pTab->aCol[iCol];
        pExpr->iTable = regBase + iCol + 1;
        pExpr->affinity = pCol->affinity;
        zColl = pCol->zColl;
        if( zColl==0 ) zColl = db->pDfltColl->zName;
        pExpr = sqlite3ExprAddCollateString(pParse, pExpr, zColl);
      }else{
        pExpr->iTable = regBase;
        pExpr->affinity = SQLITE_AFF_INTEGER;
      }
    }
    return pExpr;
  }
  
  /*
  ** Return an Expr object that refers to column iCol of table pTab which
  ** has cursor iCur.
  */
  static Expr *exprTableColumn(
    sqlite3 *db,      /* The database connection */
    Table *pTab,      /* The table whose column is desired */
    int iCursor,      /* The open cursor on the table */
    i16 iCol          /* The column that is wanted */
  ){
    Expr *pExpr = sqlite3Expr(db, TK_COLUMN, 0);
    if( pExpr ){
      pExpr->pTab = pTab;
      pExpr->iTable = iCursor;
      pExpr->iColumn = iCol;
    }
    return pExpr;
  }
  
  /*
  ** This function is called to generate code executed when a row is deleted
  ** from the parent table of foreign key constraint pFKey and, if pFKey is 
  ** deferred, when a row is inserted into the same table. When generating
  ** code for an SQL UPDATE operation, this function may be called twice -
  ** once to "delete" the old row and once to "insert" the new row.
  **
  ** Parameter nIncr is passed -1 when inserting a row (as this may decrease
  ** the number of FK violations in the db) or +1 when deleting one (as this
  ** may increase the number of FK constraint problems).
  **
  ** The code generated by this function scans through the rows in the child
  ** table that correspond to the parent table row being deleted or inserted.
  ** For each child row found, one of the following actions is taken:
  **
  **   Operation | FK type   | Action taken
  **   --------------------------------------------------------------------------
  **   DELETE      immediate   Increment the "immediate constraint counter".
  **                           Or, if the ON (UPDATE|DELETE) action is RESTRICT,
  **                           throw a "FOREIGN KEY constraint failed" exception.
  **
  **   INSERT      immediate   Decrement the "immediate constraint counter".
  **
  **   DELETE      deferred    Increment the "deferred constraint counter".
  **                           Or, if the ON (UPDATE|DELETE) action is RESTRICT,
  **                           throw a "FOREIGN KEY constraint failed" exception.
  **
  **   INSERT      deferred    Decrement the "deferred constraint counter".
  **
  ** These operations are identified in the comment at the top of this file 
  ** (fkey.c) as "I.2" and "D.2".
  */
  static void fkScanChildren(
    Parse *pParse,                  /* Parse context */
    SrcList *pSrc,                  /* The child table to be scanned */
    Table *pTab,                    /* The parent table */
    Index *pIdx,                    /* Index on parent covering the foreign key */
    FKey *pFKey,                    /* The foreign key linking pSrc to pTab */
    int *aiCol,                     /* Map from pIdx cols to child table cols */
    int regData,                    /* Parent row data starts here */
    int nIncr                       /* Amount to increment deferred counter by */
  ){
    sqlite3 *db = pParse->db;       /* Database handle */
    int i;                          /* Iterator variable */
    Expr *pWhere = 0;               /* WHERE clause to scan with */
    NameContext sNameContext;       /* Context used to resolve WHERE clause */
    WhereInfo *pWInfo;              /* Context used by sqlite3WhereXXX() */
    int iFkIfZero = 0;              /* Address of OP_FkIfZero */
    Vdbe *v = sqlite3GetVdbe(pParse);
  
    assert( pIdx==0 || pIdx->pTable==pTab );
    assert( pIdx==0 || pIdx->nKeyCol==pFKey->nCol );
    assert( pIdx!=0 || pFKey->nCol==1 );
    assert( pIdx!=0 || HasRowid(pTab) );
  
    if( nIncr<0 ){
      iFkIfZero = sqlite3VdbeAddOp2(v, OP_FkIfZero, pFKey->isDeferred, 0);
      VdbeCoverage(v);
    }
  
    /* Create an Expr object representing an SQL expression like:
    **
    **   <parent-key1> = <child-key1> AND <parent-key2> = <child-key2> ...
    **
    ** The collation sequence used for the comparison should be that of
    ** the parent key columns. The affinity of the parent key column should
    ** be applied to each child key value before the comparison takes place.
    */
    for(i=0; i<pFKey->nCol; i++){
      Expr *pLeft;                  /* Value from parent table row */
      Expr *pRight;                 /* Column ref to child table */
      Expr *pEq;                    /* Expression (pLeft = pRight) */
      i16 iCol;                     /* Index of column in child table */ 
      const char *zCol;             /* Name of column in child table */
  
      iCol = pIdx ? pIdx->aiColumn[i] : -1;
      pLeft = exprTableRegister(pParse, pTab, regData, iCol);
      iCol = aiCol ? aiCol[i] : pFKey->aCol[0].iFrom;
      assert( iCol>=0 );
      zCol = pFKey->pFrom->aCol[iCol].zName;
      pRight = sqlite3Expr(db, TK_ID, zCol);
      pEq = sqlite3PExpr(pParse, TK_EQ, pLeft, pRight, 0);
      pWhere = sqlite3ExprAnd(db, pWhere, pEq);
    }
  
    /* If the child table is the same as the parent table, then add terms
    ** to the WHERE clause that prevent this entry from being scanned.
    ** The added WHERE clause terms are like this:
    **
    **     $current_rowid!=rowid
    **     NOT( $current_a==a AND $current_b==b AND ... )
    **
    ** The first form is used for rowid tables.  The second form is used
    ** for WITHOUT ROWID tables.  In the second form, the primary key is
    ** (a,b,...)
    */
    if( pTab==pFKey->pFrom && nIncr>0 ){
      Expr *pNe;                    /* Expression (pLeft != pRight) */
      Expr *pLeft;                  /* Value from parent table row */
      Expr *pRight;                 /* Column ref to child table */
      if( HasRowid(pTab) ){
        pLeft = exprTableRegister(pParse, pTab, regData, -1);
        pRight = exprTableColumn(db, pTab, pSrc->a[0].iCursor, -1);
        pNe = sqlite3PExpr(pParse, TK_NE, pLeft, pRight, 0);
      }else{
        Expr *pEq, *pAll = 0;
        Index *pPk = sqlite3PrimaryKeyIndex(pTab);
        assert( pIdx!=0 );
        for(i=0; i<pPk->nKeyCol; i++){
          i16 iCol = pIdx->aiColumn[i];
          assert( iCol>=0 );
          pLeft = exprTableRegister(pParse, pTab, regData, iCol);
          pRight = exprTableColumn(db, pTab, pSrc->a[0].iCursor, iCol);
          pEq = sqlite3PExpr(pParse, TK_EQ, pLeft, pRight, 0);
          pAll = sqlite3ExprAnd(db, pAll, pEq);
        }
        pNe = sqlite3PExpr(pParse, TK_NOT, pAll, 0, 0);
      }
      pWhere = sqlite3ExprAnd(db, pWhere, pNe);
    }
  
    /* Resolve the references in the WHERE clause. */
    memset(&sNameContext, 0, sizeof(NameContext));
    sNameContext.pSrcList = pSrc;
    sNameContext.pParse = pParse;
    sqlite3ResolveExprNames(&sNameContext, pWhere);
  
    /* Create VDBE to loop through the entries in pSrc that match the WHERE
    ** clause. For each row found, increment either the deferred or immediate
    ** foreign key constraint counter. */
    pWInfo = sqlite3WhereBegin(pParse, pSrc, pWhere, 0, 0, 0, 0);
    sqlite3VdbeAddOp2(v, OP_FkCounter, pFKey->isDeferred, nIncr);
    if( pWInfo ){
      sqlite3WhereEnd(pWInfo);
    }
  
    /* Clean up the WHERE clause constructed above. */
    sqlite3ExprDelete(db, pWhere);
    if( iFkIfZero ){
      sqlite3VdbeJumpHere(v, iFkIfZero);
    }
  }
  
  /*
  ** This function returns a linked list of FKey objects (connected by
  ** FKey.pNextTo) holding all children of table pTab.  For example,
  ** given the following schema:
  **
  **   CREATE TABLE t1(a PRIMARY KEY);
  **   CREATE TABLE t2(b REFERENCES t1(a);
  **
  ** Calling this function with table "t1" as an argument returns a pointer
  ** to the FKey structure representing the foreign key constraint on table
  ** "t2". Calling this function with "t2" as the argument would return a
  ** NULL pointer (as there are no FK constraints for which t2 is the parent
  ** table).
  */
  FKey *sqlite3FkReferences(Table *pTab){
    return (FKey *)sqlite3HashFind(&pTab->pSchema->fkeyHash, pTab->zName);
  }
  
  /*
  ** The second argument is a Trigger structure allocated by the 
  ** fkActionTrigger() routine. This function deletes the Trigger structure
  ** and all of its sub-components.
  **
  ** The Trigger structure or any of its sub-components may be allocated from
  ** the lookaside buffer belonging to database handle dbMem.
  */
  static void fkTriggerDelete(sqlite3 *dbMem, Trigger *p){
    if( p ){
      TriggerStep *pStep = p->step_list;
      sqlite3ExprDelete(dbMem, pStep->pWhere);
      sqlite3ExprListDelete(dbMem, pStep->pExprList);
      sqlite3SelectDelete(dbMem, pStep->pSelect);
      sqlite3ExprDelete(dbMem, p->pWhen);
      sqlite3DbFree(dbMem, p);
    }
  }
  
  /*
  ** This function is called to generate code that runs when table pTab is
  ** being dropped from the database. The SrcList passed as the second argument
  ** to this function contains a single entry guaranteed to resolve to
  ** table pTab.
  **
  ** Normally, no code is required. However, if either
  **
  **   (a) The table is the parent table of a FK constraint, or
  **   (b) The table is the child table of a deferred FK constraint and it is
  **       determined at runtime that there are outstanding deferred FK 
  **       constraint violations in the database,
  **
  ** then the equivalent of "DELETE FROM <tbl>" is executed before dropping
  ** the table from the database. Triggers are disabled while running this
  ** DELETE, but foreign key actions are not.
  */
  void sqlite3FkDropTable(Parse *pParse, SrcList *pName, Table *pTab){
    sqlite3 *db = pParse->db;
    if( (db->flags&SQLITE_ForeignKeys) && !IsVirtual(pTab) && !pTab->pSelect ){
      int iSkip = 0;
      Vdbe *v = sqlite3GetVdbe(pParse);
  
      assert( v );                  /* VDBE has already been allocated */
      if( sqlite3FkReferences(pTab)==0 ){
        /* Search for a deferred foreign key constraint for which this table
        ** is the child table. If one cannot be found, return without 
        ** generating any VDBE code. If one can be found, then jump over
        ** the entire DELETE if there are no outstanding deferred constraints
        ** when this statement is run.  */
        FKey *p;
        for(p=pTab->pFKey; p; p=p->pNextFrom){
          if( p->isDeferred || (db->flags & SQLITE_DeferFKs) ) break;
        }
        if( !p ) return;
        iSkip = sqlite3VdbeMakeLabel(v);
        sqlite3VdbeAddOp2(v, OP_FkIfZero, 1, iSkip); VdbeCoverage(v);
      }
  
      pParse->disableTriggers = 1;
      sqlite3DeleteFrom(pParse, sqlite3SrcListDup(db, pName, 0), 0, 0, 0, 0);
      pParse->disableTriggers = 0;
  
      /* If the DELETE has generated immediate foreign key constraint 
      ** violations, halt the VDBE and return an error at this point, before
      ** any modifications to the schema are made. This is because statement
      ** transactions are not able to rollback schema changes.  
      **
      ** If the SQLITE_DeferFKs flag is set, then this is not required, as
      ** the statement transaction will not be rolled back even if FK
      ** constraints are violated.
      */
      if( (db->flags & SQLITE_DeferFKs)==0 ){
        sqlite3VdbeAddOp2(v, OP_FkIfZero, 0, sqlite3VdbeCurrentAddr(v)+2);
        VdbeCoverage(v);
        sqlite3HaltConstraint(pParse, SQLITE_CONSTRAINT_FOREIGNKEY,
            OE_Abort, 0, P4_STATIC, P5_ConstraintFK);
      }
  
      if( iSkip ){
        sqlite3VdbeResolveLabel(v, iSkip);
      }
    }
  }
  
  
  /*
  ** The second argument points to an FKey object representing a foreign key
  ** for which pTab is the child table. An UPDATE statement against pTab
  ** is currently being processed. For each column of the table that is 
  ** actually updated, the corresponding element in the aChange[] array
  ** is zero or greater (if a column is unmodified the corresponding element
  ** is set to -1). If the rowid column is modified by the UPDATE statement
  ** the bChngRowid argument is non-zero.
  **
  ** This function returns true if any of the columns that are part of the
  ** child key for FK constraint *p are modified.
  */
  static int fkChildIsModified(
    Table *pTab,                    /* Table being updated */
    FKey *p,                        /* Foreign key for which pTab is the child */
    int *aChange,                   /* Array indicating modified columns */
    int bChngRowid                  /* True if rowid is modified by this update */
  ){
    int i;
    for(i=0; i<p->nCol; i++){
      int iChildKey = p->aCol[i].iFrom;
      if( aChange[iChildKey]>=0 ) return 1;
      if( iChildKey==pTab->iPKey && bChngRowid ) return 1;
    }
    return 0;
  }
  
  /*
  ** The second argument points to an FKey object representing a foreign key
  ** for which pTab is the parent table. An UPDATE statement against pTab
  ** is currently being processed. For each column of the table that is 
  ** actually updated, the corresponding element in the aChange[] array
  ** is zero or greater (if a column is unmodified the corresponding element
  ** is set to -1). If the rowid column is modified by the UPDATE statement
  ** the bChngRowid argument is non-zero.
  **
  ** This function returns true if any of the columns that are part of the
  ** parent key for FK constraint *p are modified.
  */
  static int fkParentIsModified(
    Table *pTab, 
    FKey *p, 
    int *aChange, 
    int bChngRowid
  ){
    int i;
    for(i=0; i<p->nCol; i++){
      char *zKey = p->aCol[i].zCol;
      int iKey;
      for(iKey=0; iKey<pTab->nCol; iKey++){
        if( aChange[iKey]>=0 || (iKey==pTab->iPKey && bChngRowid) ){
          Column *pCol = &pTab->aCol[iKey];
          if( zKey ){
            if( 0==sqlite3StrICmp(pCol->zName, zKey) ) return 1;
          }else if( pCol->colFlags & COLFLAG_PRIMKEY ){
            return 1;
          }
        }
      }
    }
    return 0;
  }
  
  /*
  ** Return true if the parser passed as the first argument is being
  ** used to code a trigger that is really a "SET NULL" action belonging
  ** to trigger pFKey.
  */
  static int isSetNullAction(Parse *pParse, FKey *pFKey){
    Parse *pTop = sqlite3ParseToplevel(pParse);
    if( pTop->pTriggerPrg ){
      Trigger *p = pTop->pTriggerPrg->pTrigger;
      if( (p==pFKey->apTrigger[0] && pFKey->aAction[0]==OE_SetNull)
       || (p==pFKey->apTrigger[1] && pFKey->aAction[1]==OE_SetNull)
      ){
        return 1;
      }
    }
    return 0;
  }
  
  /*
  ** This function is called when inserting, deleting or updating a row of
  ** table pTab to generate VDBE code to perform foreign key constraint 
  ** processing for the operation.
  **
  ** For a DELETE operation, parameter regOld is passed the index of the
  ** first register in an array of (pTab->nCol+1) registers containing the
  ** rowid of the row being deleted, followed by each of the column values
  ** of the row being deleted, from left to right. Parameter regNew is passed
  ** zero in this case.
  **
  ** For an INSERT operation, regOld is passed zero and regNew is passed the
  ** first register of an array of (pTab->nCol+1) registers containing the new
  ** row data.
  **
  ** For an UPDATE operation, this function is called twice. Once before
  ** the original record is deleted from the table using the calling convention
  ** described for DELETE. Then again after the original record is deleted
  ** but before the new record is inserted using the INSERT convention. 
  */
  void sqlite3FkCheck(
    Parse *pParse,                  /* Parse context */
    Table *pTab,                    /* Row is being deleted from this table */ 
    int regOld,                     /* Previous row data is stored here */
    int regNew,                     /* New row data is stored here */
    int *aChange,                   /* Array indicating UPDATEd columns (or 0) */
    int bChngRowid                  /* True if rowid is UPDATEd */
  ){
    sqlite3 *db = pParse->db;       /* Database handle */
    FKey *pFKey;                    /* Used to iterate through FKs */
    int iDb;                        /* Index of database containing pTab */
    const char *zDb;                /* Name of database containing pTab */
    int isIgnoreErrors = pParse->disableTriggers;
  
    /* Exactly one of regOld and regNew should be non-zero. */
    assert( (regOld==0)!=(regNew==0) );
  
    /* If foreign-keys are disabled, this function is a no-op. */
    if( (db->flags&SQLITE_ForeignKeys)==0 ) return;
  
    iDb = sqlite3SchemaToIndex(db, pTab->pSchema);
    zDb = db->aDb[iDb].zDbSName;
  
    /* Loop through all the foreign key constraints for which pTab is the
    ** child table (the table that the foreign key definition is part of).  */
    for(pFKey=pTab->pFKey; pFKey; pFKey=pFKey->pNextFrom){
      Table *pTo;                   /* Parent table of foreign key pFKey */
      Index *pIdx = 0;              /* Index on key columns in pTo */
      int *aiFree = 0;
      int *aiCol;
      int iCol;
      int i;
      int bIgnore = 0;
  
      if( aChange 
       && sqlite3_stricmp(pTab->zName, pFKey->zTo)!=0
       && fkChildIsModified(pTab, pFKey, aChange, bChngRowid)==0 
      ){
        continue;
      }
  
      /* Find the parent table of this foreign key. Also find a unique index 
      ** on the parent key columns in the parent table. If either of these 
      ** schema items cannot be located, set an error in pParse and return 
      ** early.  */
      if( pParse->disableTriggers ){
        pTo = sqlite3FindTable(db, pFKey->zTo, zDb);
      }else{
        pTo = sqlite3LocateTable(pParse, 0, pFKey->zTo, zDb);
      }
      if( !pTo || sqlite3FkLocateIndex(pParse, pTo, pFKey, &pIdx, &aiFree) ){
        assert( isIgnoreErrors==0 || (regOld!=0 && regNew==0) );
        if( !isIgnoreErrors || db->mallocFailed ) return;
        if( pTo==0 ){
          /* If isIgnoreErrors is true, then a table is being dropped. In this
          ** case SQLite runs a "DELETE FROM xxx" on the table being dropped
          ** before actually dropping it in order to check FK constraints.
          ** If the parent table of an FK constraint on the current table is
          ** missing, behave as if it is empty. i.e. decrement the relevant
          ** FK counter for each row of the current table with non-NULL keys.
          */
          Vdbe *v = sqlite3GetVdbe(pParse);
          int iJump = sqlite3VdbeCurrentAddr(v) + pFKey->nCol + 1;
          for(i=0; i<pFKey->nCol; i++){
            int iReg = pFKey->aCol[i].iFrom + regOld + 1;
            sqlite3VdbeAddOp2(v, OP_IsNull, iReg, iJump); VdbeCoverage(v);
          }
          sqlite3VdbeAddOp2(v, OP_FkCounter, pFKey->isDeferred, -1);
        }
        continue;
      }
      assert( pFKey->nCol==1 || (aiFree && pIdx) );
  
      if( aiFree ){
        aiCol = aiFree;
      }else{
        iCol = pFKey->aCol[0].iFrom;
        aiCol = &iCol;
      }
      for(i=0; i<pFKey->nCol; i++){
        if( aiCol[i]==pTab->iPKey ){
          aiCol[i] = -1;
        }
        assert( pIdx==0 || pIdx->aiColumn[i]>=0 );
  #ifndef SQLITE_OMIT_AUTHORIZATION
        /* Request permission to read the parent key columns. If the 
        ** authorization callback returns SQLITE_IGNORE, behave as if any
        ** values read from the parent table are NULL. */
        if( db->xAuth ){
          int rcauth;
          char *zCol = pTo->aCol[pIdx ? pIdx->aiColumn[i] : pTo->iPKey].zName;
          rcauth = sqlite3AuthReadCol(pParse, pTo->zName, zCol, iDb);
          bIgnore = (rcauth==SQLITE_IGNORE);
        }
  #endif
      }
  
      /* Take a shared-cache advisory read-lock on the parent table. Allocate 
      ** a cursor to use to search the unique index on the parent key columns 
      ** in the parent table.  */
      sqlite3TableLock(pParse, iDb, pTo->tnum, 0, pTo->zName);
      pParse->nTab++;
  
      if( regOld!=0 ){
        /* A row is being removed from the child table. Search for the parent.
        ** If the parent does not exist, removing the child row resolves an 
        ** outstanding foreign key constraint violation. */
        fkLookupParent(pParse, iDb, pTo, pIdx, pFKey, aiCol, regOld, -1, bIgnore);
      }
      if( regNew!=0 && !isSetNullAction(pParse, pFKey) ){
        /* A row is being added to the child table. If a parent row cannot
        ** be found, adding the child row has violated the FK constraint. 
        **
        ** If this operation is being performed as part of a trigger program
        ** that is actually a "SET NULL" action belonging to this very 
        ** foreign key, then omit this scan altogether. As all child key
        ** values are guaranteed to be NULL, it is not possible for adding
        ** this row to cause an FK violation.  */
        fkLookupParent(pParse, iDb, pTo, pIdx, pFKey, aiCol, regNew, +1, bIgnore);
      }
  
      sqlite3DbFree(db, aiFree);
    }
  
    /* Loop through all the foreign key constraints that refer to this table.
    ** (the "child" constraints) */
    for(pFKey = sqlite3FkReferences(pTab); pFKey; pFKey=pFKey->pNextTo){
      Index *pIdx = 0;              /* Foreign key index for pFKey */
      SrcList *pSrc;
      int *aiCol = 0;
  
      if( aChange && fkParentIsModified(pTab, pFKey, aChange, bChngRowid)==0 ){
        continue;
      }
  
      if( !pFKey->isDeferred && !(db->flags & SQLITE_DeferFKs) 
       && !pParse->pToplevel && !pParse->isMultiWrite 
      ){
        assert( regOld==0 && regNew!=0 );
        /* Inserting a single row into a parent table cannot cause (or fix)
        ** an immediate foreign key violation. So do nothing in this case.  */
        continue;
      }
  
      if( sqlite3FkLocateIndex(pParse, pTab, pFKey, &pIdx, &aiCol) ){
        if( !isIgnoreErrors || db->mallocFailed ) return;
        continue;
      }
      assert( aiCol || pFKey->nCol==1 );
  
      /* Create a SrcList structure containing the child table.  We need the
      ** child table as a SrcList for sqlite3WhereBegin() */
      pSrc = sqlite3SrcListAppend(db, 0, 0, 0);
      if( pSrc ){
        struct SrcList_item *pItem = pSrc->a;
        pItem->pTab = pFKey->pFrom;
        pItem->zName = pFKey->pFrom->zName;
        pItem->pTab->nRef++;
        pItem->iCursor = pParse->nTab++;
    
        if( regNew!=0 ){
          fkScanChildren(pParse, pSrc, pTab, pIdx, pFKey, aiCol, regNew, -1);
        }
        if( regOld!=0 ){
          int eAction = pFKey->aAction[aChange!=0];
          fkScanChildren(pParse, pSrc, pTab, pIdx, pFKey, aiCol, regOld, 1);
          /* If this is a deferred FK constraint, or a CASCADE or SET NULL
          ** action applies, then any foreign key violations caused by
          ** removing the parent key will be rectified by the action trigger.
          ** So do not set the "may-abort" flag in this case.
          **
          ** Note 1: If the FK is declared "ON UPDATE CASCADE", then the
          ** may-abort flag will eventually be set on this statement anyway
          ** (when this function is called as part of processing the UPDATE
          ** within the action trigger).
          **
          ** Note 2: At first glance it may seem like SQLite could simply omit
          ** all OP_FkCounter related scans when either CASCADE or SET NULL
          ** applies. The trouble starts if the CASCADE or SET NULL action 
          ** trigger causes other triggers or action rules attached to the 
          ** child table to fire. In these cases the fk constraint counters
          ** might be set incorrectly if any OP_FkCounter related scans are 
          ** omitted.  */
          if( !pFKey->isDeferred && eAction!=OE_Cascade && eAction!=OE_SetNull ){
            sqlite3MayAbort(pParse);
          }
        }
        pItem->zName = 0;
        sqlite3SrcListDelete(db, pSrc);
      }
      sqlite3DbFree(db, aiCol);
    }
  }
  
  #define COLUMN_MASK(x) (((x)>31) ? 0xffffffff : ((u32)1<<(x)))
  
  /*
  ** This function is called before generating code to update or delete a 
  ** row contained in table pTab.
  */
  u32 sqlite3FkOldmask(
    Parse *pParse,                  /* Parse context */
    Table *pTab                     /* Table being modified */
  ){
    u32 mask = 0;
    if( pParse->db->flags&SQLITE_ForeignKeys ){
      FKey *p;
      int i;
      for(p=pTab->pFKey; p; p=p->pNextFrom){
        for(i=0; i<p->nCol; i++) mask |= COLUMN_MASK(p->aCol[i].iFrom);
      }
      for(p=sqlite3FkReferences(pTab); p; p=p->pNextTo){
        Index *pIdx = 0;
        sqlite3FkLocateIndex(pParse, pTab, p, &pIdx, 0);
        if( pIdx ){
          for(i=0; i<pIdx->nKeyCol; i++){
            assert( pIdx->aiColumn[i]>=0 );
            mask |= COLUMN_MASK(pIdx->aiColumn[i]);
          }
        }
      }
    }
    return mask;
  }
  
  
  /*
  ** This function is called before generating code to update or delete a 
  ** row contained in table pTab. If the operation is a DELETE, then
  ** parameter aChange is passed a NULL value. For an UPDATE, aChange points
  ** to an array of size N, where N is the number of columns in table pTab.
  ** If the i'th column is not modified by the UPDATE, then the corresponding 
  ** entry in the aChange[] array is set to -1. If the column is modified,
  ** the value is 0 or greater. Parameter chngRowid is set to true if the
  ** UPDATE statement modifies the rowid fields of the table.
  **
  ** If any foreign key processing will be required, this function returns
  ** true. If there is no foreign key related processing, this function 
  ** returns false.
  */
  int sqlite3FkRequired(
    Parse *pParse,                  /* Parse context */
    Table *pTab,                    /* Table being modified */
    int *aChange,                   /* Non-NULL for UPDATE operations */
    int chngRowid                   /* True for UPDATE that affects rowid */
  ){
    if( pParse->db->flags&SQLITE_ForeignKeys ){
      if( !aChange ){
        /* A DELETE operation. Foreign key processing is required if the 
        ** table in question is either the child or parent table for any 
        ** foreign key constraint.  */
        return (sqlite3FkReferences(pTab) || pTab->pFKey);
      }else{
        /* This is an UPDATE. Foreign key processing is only required if the
        ** operation modifies one or more child or parent key columns. */
        FKey *p;
  
        /* Check if any child key columns are being modified. */
        for(p=pTab->pFKey; p; p=p->pNextFrom){
          if( fkChildIsModified(pTab, p, aChange, chngRowid) ) return 1;
        }
  
        /* Check if any parent key columns are being modified. */
        for(p=sqlite3FkReferences(pTab); p; p=p->pNextTo){
          if( fkParentIsModified(pTab, p, aChange, chngRowid) ) return 1;
        }
      }
    }
    return 0;
  }
  
  /*
  ** This function is called when an UPDATE or DELETE operation is being 
  ** compiled on table pTab, which is the parent table of foreign-key pFKey.
  ** If the current operation is an UPDATE, then the pChanges parameter is
  ** passed a pointer to the list of columns being modified. If it is a
  ** DELETE, pChanges is passed a NULL pointer.
  **
  ** It returns a pointer to a Trigger structure containing a trigger
  ** equivalent to the ON UPDATE or ON DELETE action specified by pFKey.
  ** If the action is "NO ACTION" or "RESTRICT", then a NULL pointer is
  ** returned (these actions require no special handling by the triggers
  ** sub-system, code for them is created by fkScanChildren()).
  **
  ** For example, if pFKey is the foreign key and pTab is table "p" in 
  ** the following schema:
  **
  **   CREATE TABLE p(pk PRIMARY KEY);
  **   CREATE TABLE c(ck REFERENCES p ON DELETE CASCADE);
  **
  ** then the returned trigger structure is equivalent to:
  **
  **   CREATE TRIGGER ... DELETE ON p BEGIN
  **     DELETE FROM c WHERE ck = old.pk;
  **   END;
  **
  ** The returned pointer is cached as part of the foreign key object. It
  ** is eventually freed along with the rest of the foreign key object by 
  ** sqlite3FkDelete().
  */
  static Trigger *fkActionTrigger(
    Parse *pParse,                  /* Parse context */
    Table *pTab,                    /* Table being updated or deleted from */
    FKey *pFKey,                    /* Foreign key to get action for */
    ExprList *pChanges              /* Change-list for UPDATE, NULL for DELETE */
  ){
    sqlite3 *db = pParse->db;       /* Database handle */
    int action;                     /* One of OE_None, OE_Cascade etc. */
    Trigger *pTrigger;              /* Trigger definition to return */
    int iAction = (pChanges!=0);    /* 1 for UPDATE, 0 for DELETE */
  
    action = pFKey->aAction[iAction];
    if( action==OE_Restrict && (db->flags & SQLITE_DeferFKs) ){
      return 0;
    }
    pTrigger = pFKey->apTrigger[iAction];
  
    if( action!=OE_None && !pTrigger ){
      char const *zFrom;            /* Name of child table */
      int nFrom;                    /* Length in bytes of zFrom */
      Index *pIdx = 0;              /* Parent key index for this FK */
      int *aiCol = 0;               /* child table cols -> parent key cols */
      TriggerStep *pStep = 0;        /* First (only) step of trigger program */
      Expr *pWhere = 0;             /* WHERE clause of trigger step */
      ExprList *pList = 0;          /* Changes list if ON UPDATE CASCADE */
      Select *pSelect = 0;          /* If RESTRICT, "SELECT RAISE(...)" */
      int i;                        /* Iterator variable */
      Expr *pWhen = 0;              /* WHEN clause for the trigger */
  
      if( sqlite3FkLocateIndex(pParse, pTab, pFKey, &pIdx, &aiCol) ) return 0;
      assert( aiCol || pFKey->nCol==1 );
  
      for(i=0; i<pFKey->nCol; i++){
        Token tOld = { "old", 3 };  /* Literal "old" token */
        Token tNew = { "new", 3 };  /* Literal "new" token */
        Token tFromCol;             /* Name of column in child table */
        Token tToCol;               /* Name of column in parent table */
        int iFromCol;               /* Idx of column in child table */
        Expr *pEq;                  /* tFromCol = OLD.tToCol */
  
        iFromCol = aiCol ? aiCol[i] : pFKey->aCol[0].iFrom;
        assert( iFromCol>=0 );
        assert( pIdx!=0 || (pTab->iPKey>=0 && pTab->iPKey<pTab->nCol) );
        assert( pIdx==0 || pIdx->aiColumn[i]>=0 );
        sqlite3TokenInit(&tToCol,
                     pTab->aCol[pIdx ? pIdx->aiColumn[i] : pTab->iPKey].zName);
        sqlite3TokenInit(&tFromCol, pFKey->pFrom->aCol[iFromCol].zName);
  
        /* Create the expression "OLD.zToCol = zFromCol". It is important
        ** that the "OLD.zToCol" term is on the LHS of the = operator, so
        ** that the affinity and collation sequence associated with the
        ** parent table are used for the comparison. */
        pEq = sqlite3PExpr(pParse, TK_EQ,
            sqlite3PExpr(pParse, TK_DOT, 
              sqlite3ExprAlloc(db, TK_ID, &tOld, 0),
              sqlite3ExprAlloc(db, TK_ID, &tToCol, 0)
            , 0),
            sqlite3ExprAlloc(db, TK_ID, &tFromCol, 0)
        , 0);
        pWhere = sqlite3ExprAnd(db, pWhere, pEq);
  
        /* For ON UPDATE, construct the next term of the WHEN clause.
        ** The final WHEN clause will be like this:
        **
        **    WHEN NOT(old.col1 IS new.col1 AND ... AND old.colN IS new.colN)
        */
        if( pChanges ){
          pEq = sqlite3PExpr(pParse, TK_IS,
              sqlite3PExpr(pParse, TK_DOT, 
                sqlite3ExprAlloc(db, TK_ID, &tOld, 0),
                sqlite3ExprAlloc(db, TK_ID, &tToCol, 0),
                0),
              sqlite3PExpr(pParse, TK_DOT, 
                sqlite3ExprAlloc(db, TK_ID, &tNew, 0),
                sqlite3ExprAlloc(db, TK_ID, &tToCol, 0),
                0),
              0);
          pWhen = sqlite3ExprAnd(db, pWhen, pEq);
        }
    
        if( action!=OE_Restrict && (action!=OE_Cascade || pChanges) ){
          Expr *pNew;
          if( action==OE_Cascade ){
            pNew = sqlite3PExpr(pParse, TK_DOT, 
              sqlite3ExprAlloc(db, TK_ID, &tNew, 0),
              sqlite3ExprAlloc(db, TK_ID, &tToCol, 0)
            , 0);
          }else if( action==OE_SetDflt ){
            Expr *pDflt = pFKey->pFrom->aCol[iFromCol].pDflt;
            if( pDflt ){
              pNew = sqlite3ExprDup(db, pDflt, 0);
            }else{
              pNew = sqlite3ExprAlloc(db, TK_NULL, 0, 0);
            }
          }else{
            pNew = sqlite3ExprAlloc(db, TK_NULL, 0, 0);
          }
          pList = sqlite3ExprListAppend(pParse, pList, pNew);
          sqlite3ExprListSetName(pParse, pList, &tFromCol, 0);
        }
      }
      sqlite3DbFree(db, aiCol);
  
      zFrom = pFKey->pFrom->zName;
      nFrom = sqlite3Strlen30(zFrom);
  
      if( action==OE_Restrict ){
        Token tFrom;
        Expr *pRaise; 
  
        tFrom.z = zFrom;
        tFrom.n = nFrom;
        pRaise = sqlite3Expr(db, TK_RAISE, "FOREIGN KEY constraint failed");
        if( pRaise ){
          pRaise->affinity = OE_Abort;
        }
        pSelect = sqlite3SelectNew(pParse, 
            sqlite3ExprListAppend(pParse, 0, pRaise),
            sqlite3SrcListAppend(db, 0, &tFrom, 0),
            pWhere,
            0, 0, 0, 0, 0, 0
        );
        pWhere = 0;
      }
  
      /* Disable lookaside memory allocation */
      db->lookaside.bDisable++;
  
      pTrigger = (Trigger *)sqlite3DbMallocZero(db, 
          sizeof(Trigger) +         /* struct Trigger */
          sizeof(TriggerStep) +     /* Single step in trigger program */
          nFrom + 1                 /* Space for pStep->zTarget */
      );
      if( pTrigger ){
        pStep = pTrigger->step_list = (TriggerStep *)&pTrigger[1];
        pStep->zTarget = (char *)&pStep[1];
        memcpy((char *)pStep->zTarget, zFrom, nFrom);
    
        pStep->pWhere = sqlite3ExprDup(db, pWhere, EXPRDUP_REDUCE);
        pStep->pExprList = sqlite3ExprListDup(db, pList, EXPRDUP_REDUCE);
        pStep->pSelect = sqlite3SelectDup(db, pSelect, EXPRDUP_REDUCE);
        if( pWhen ){
          pWhen = sqlite3PExpr(pParse, TK_NOT, pWhen, 0, 0);
          pTrigger->pWhen = sqlite3ExprDup(db, pWhen, EXPRDUP_REDUCE);
        }
      }
  
      /* Re-enable the lookaside buffer, if it was disabled earlier. */
      db->lookaside.bDisable--;
  
      sqlite3ExprDelete(db, pWhere);
      sqlite3ExprDelete(db, pWhen);
      sqlite3ExprListDelete(db, pList);
      sqlite3SelectDelete(db, pSelect);
      if( db->mallocFailed==1 ){
        fkTriggerDelete(db, pTrigger);
        return 0;
      }
      assert( pStep!=0 );
  
      switch( action ){
        case OE_Restrict:
          pStep->op = TK_SELECT; 
          break;
        case OE_Cascade: 
          if( !pChanges ){ 
            pStep->op = TK_DELETE; 
            break; 
          }
        default:
          pStep->op = TK_UPDATE;
      }
      pStep->pTrig = pTrigger;
      pTrigger->pSchema = pTab->pSchema;
      pTrigger->pTabSchema = pTab->pSchema;
      pFKey->apTrigger[iAction] = pTrigger;
      pTrigger->op = (pChanges ? TK_UPDATE : TK_DELETE);
    }
  
    return pTrigger;
  }
  
  /*
  ** This function is called when deleting or updating a row to implement
  ** any required CASCADE, SET NULL or SET DEFAULT actions.
  */
  void sqlite3FkActions(
    Parse *pParse,                  /* Parse context */
    Table *pTab,                    /* Table being updated or deleted from */
    ExprList *pChanges,             /* Change-list for UPDATE, NULL for DELETE */
    int regOld,                     /* Address of array containing old row */
    int *aChange,                   /* Array indicating UPDATEd columns (or 0) */
    int bChngRowid                  /* True if rowid is UPDATEd */
  ){
    /* If foreign-key support is enabled, iterate through all FKs that 
    ** refer to table pTab. If there is an action associated with the FK 
    ** for this operation (either update or delete), invoke the associated 
    ** trigger sub-program.  */
    if( pParse->db->flags&SQLITE_ForeignKeys ){
      FKey *pFKey;                  /* Iterator variable */
      for(pFKey = sqlite3FkReferences(pTab); pFKey; pFKey=pFKey->pNextTo){
        if( aChange==0 || fkParentIsModified(pTab, pFKey, aChange, bChngRowid) ){
          Trigger *pAct = fkActionTrigger(pParse, pTab, pFKey, pChanges);
          if( pAct ){
            sqlite3CodeRowTriggerDirect(pParse, pAct, pTab, regOld, OE_Abort, 0);
          }
        }
      }
    }
  }
  
  #endif /* ifndef SQLITE_OMIT_TRIGGER */
  
  /*
  ** Free all memory associated with foreign key definitions attached to
  ** table pTab. Remove the deleted foreign keys from the Schema.fkeyHash
  ** hash table.
  */
  void sqlite3FkDelete(sqlite3 *db, Table *pTab){
    FKey *pFKey;                    /* Iterator variable */
    FKey *pNext;                    /* Copy of pFKey->pNextFrom */
  
    assert( db==0 || IsVirtual(pTab)
           || sqlite3SchemaMutexHeld(db, 0, pTab->pSchema) );
    for(pFKey=pTab->pFKey; pFKey; pFKey=pNext){
  
      /* Remove the FK from the fkeyHash hash table. */
      if( !db || db->pnBytesFreed==0 ){
        if( pFKey->pPrevTo ){
          pFKey->pPrevTo->pNextTo = pFKey->pNextTo;
        }else{
          void *p = (void *)pFKey->pNextTo;
          const char *z = (p ? pFKey->pNextTo->zTo : pFKey->zTo);
          sqlite3HashInsert(&pTab->pSchema->fkeyHash, z, p);
        }
        if( pFKey->pNextTo ){
          pFKey->pNextTo->pPrevTo = pFKey->pPrevTo;
        }
      }
  
      /* EV: R-30323-21917 Each foreign key constraint in SQLite is
      ** classified as either immediate or deferred.
      */
      assert( pFKey->isDeferred==0 || pFKey->isDeferred==1 );
  
      /* Delete any triggers created to implement actions for this FK. */
  #ifndef SQLITE_OMIT_TRIGGER
      fkTriggerDelete(db, pFKey->apTrigger[0]);
      fkTriggerDelete(db, pFKey->apTrigger[1]);
  #endif
  
      pNext = pFKey->pNextFrom;
      sqlite3DbFree(db, pFKey);
    }
  }
  #endif /* ifndef SQLITE_OMIT_FOREIGN_KEY */
  