xref: /sqlite-3.40.0/src/fkey.c (revision 41ce47c4)

/*
**
** 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].zCnName, 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->pPartIdxWhere==0 ){
      /* 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 = sqlite3ColumnColl(&pParent->aCol[iCol]);
          if( !zDfltColl ) zDfltColl = sqlite3StrBINARY;
          if( sqlite3StrICmp(pIdx->azColl[i], zDfltColl) ) break;

          zIdxCol = pParent->aCol[iCol].zCnName;
          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(pParse);   /* jump here if parent key found */

  sqlite3VdbeVerifyAbortable(v,
    (!pFKey->isDeferred
      && !(pParse->db->flags & SQLITE_DeferFKs)
      && !pParse->pToplevel
      && !pParse->isMultiWrite) ? OE_Abort : OE_Ignore);

  /* 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 = sqlite3TableColumnToStorage(pFKey->pFrom,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,
        sqlite3TableColumnToStorage(pFKey->pFrom,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);

      sqlite3VdbeAddOp3(v, OP_OpenRead, iCur, pIdx->tnum, iDb);
      sqlite3VdbeSetP4KeyInfo(pParse, pIdx);
      for(i=0; i<nCol; i++){
        sqlite3VdbeAddOp2(v, OP_Copy,
               sqlite3TableColumnToStorage(pFKey->pFrom, 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 = sqlite3TableColumnToStorage(pFKey->pFrom,aiCol[i])
                              +1+regData;
          int iParent = 1+regData;
          iParent += sqlite3TableColumnToStorage(pIdx->pTable,
                                                 pIdx->aiColumn[i]);
          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_Affinity, regTemp, nCol, 0,
                        sqlite3IndexAffinityStr(pParse->db,pIdx), nCol);
      sqlite3VdbeAddOp4Int(v, OP_Found, iCur, iOk, regTemp, nCol);
      VdbeCoverage(v);
      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 + sqlite3TableColumnToStorage(pTab,iCol) + 1;
      pExpr->affExpr = pCol->affinity;
      zColl = sqlite3ColumnColl(pCol);
      if( zColl==0 ) zColl = db->pDfltColl->zName;
      pExpr = sqlite3ExprAddCollateString(pParse, pExpr, zColl);
    }else{
      pExpr->iTable = regBase;
      pExpr->affExpr = 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 ){
    assert( ExprUseYTab(pExpr) );
    pExpr->y.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".
**
**   INSERT      immediate   Decrement the "immediate constraint counter".
**
**   DELETE      deferred    Increment the "deferred constraint counter".
**
**   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].zCnName;
    pRight = sqlite3Expr(db, TK_ID, zCol);
    pEq = sqlite3PExpr(pParse, TK_EQ, pLeft, pRight);
    pWhere = sqlite3ExprAnd(pParse, 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 *parent* key is
  ** (a,b,...). Either the parent or primary key could be used to
  ** uniquely identify the current row, but the parent key is more convenient
  ** as the required values have already been loaded into registers
  ** by the caller.
  */
  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);
    }else{
      Expr *pEq, *pAll = 0;
      assert( pIdx!=0 );
      for(i=0; i<pIdx->nKeyCol; i++){
        i16 iCol = pIdx->aiColumn[i];
        assert( iCol>=0 );
        pLeft = exprTableRegister(pParse, pTab, regData, iCol);
        pRight = sqlite3Expr(db, TK_ID, pTab->aCol[iCol].zCnName);
        pEq = sqlite3PExpr(pParse, TK_IS, pLeft, pRight);
        pAll = sqlite3ExprAnd(pParse, pAll, pEq);
      }
      pNe = sqlite3PExpr(pParse, TK_NOT, pAll, 0);
    }
    pWhere = sqlite3ExprAnd(pParse, 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. */
  if( pParse->nErr==0 ){
    pWInfo = sqlite3WhereBegin(pParse, pSrc, pWhere, 0, 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 ){
    sqlite3VdbeJumpHereOrPopInst(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);
  }
}

/*
** Clear the apTrigger[] cache of CASCADE triggers for all foreign keys
** in a particular database.  This needs to happen when the schema
** changes.
*/
void sqlite3FkClearTriggerCache(sqlite3 *db, int iDb){
  HashElem *k;
  Hash *pHash = &db->aDb[iDb].pSchema->tblHash;
  for(k=sqliteHashFirst(pHash); k; k=sqliteHashNext(k)){
    Table *pTab = sqliteHashData(k);
    FKey *pFKey;
    if( !IsOrdinaryTable(pTab) ) continue;
    for(pFKey=pTab->u.tab.pFKey; pFKey; pFKey=pFKey->pNextFrom){
      fkTriggerDelete(db, pFKey->apTrigger[0]); pFKey->apTrigger[0] = 0;
      fkTriggerDelete(db, pFKey->apTrigger[1]); pFKey->apTrigger[1] = 0;
    }
  }
}

/*
** 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) && IsOrdinaryTable(pTab) ){
    int iSkip = 0;
    Vdbe *v = sqlite3GetVdbe(pParse);

    assert( v );                  /* VDBE has already been allocated */
    assert( IsOrdinaryTable(pTab) );
    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->u.tab.pFKey; p; p=p->pNextFrom){
        if( p->isDeferred || (db->flags & SQLITE_DeferFKs) ) break;
      }
      if( !p ) return;
      iSkip = sqlite3VdbeMakeLabel(pParse);
      sqlite3VdbeAddOp2(v, OP_FkIfZero, 1, iSkip); VdbeCoverage(v);
    }

    pParse->disableTriggers = 1;
    sqlite3DeleteFrom(pParse, sqlite3SrcListDup(db, pName, 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 ){
      sqlite3VdbeVerifyAbortable(v, OE_Abort);
      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->zCnName, 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;
  if( !IsOrdinaryTable(pTab) ) 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->u.tab.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 iFromCol, iReg;
          iFromCol = pFKey->aCol[i].iFrom;
          iReg = sqlite3TableColumnToStorage(pFKey->pFrom,iFromCol) + 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].zCnName;
        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(pParse, 0, 0, 0);
    if( pSrc ){
      SrcItem *pItem = pSrc->a;
      pItem->pTab = pFKey->pFrom;
      pItem->zName = pFKey->pFrom->zName;
      pItem->pTab->nTabRef++;
      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 && IsOrdinaryTable(pTab) ){
    FKey *p;
    int i;
    for(p=pTab->u.tab.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
** non-zero. If there is no foreign key related processing, this function
** returns zero.
**
** For an UPDATE, this function returns 2 if:
**
**   * There are any FKs for which pTab is the child and the parent table
**     and any FK processing at all is required (even of a different FK), or
**
**   * the UPDATE modifies one or more parent keys for which the action is
**     not "NO ACTION" (i.e. is CASCADE, SET DEFAULT or SET NULL).
**
** Or, assuming some other foreign key processing is required, 1.
*/
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 */
){
  int eRet = 1;                   /* Value to return if bHaveFK is true */
  int bHaveFK = 0;                /* If FK processing is required */
  if( pParse->db->flags&SQLITE_ForeignKeys && IsOrdinaryTable(pTab) ){
    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.  */
      bHaveFK = (sqlite3FkReferences(pTab) || pTab->u.tab.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->u.tab.pFKey; p; p=p->pNextFrom){
        if( fkChildIsModified(pTab, p, aChange, chngRowid) ){
          if( 0==sqlite3_stricmp(pTab->zName, p->zTo) ) eRet = 2;
          bHaveFK = 1;
        }
      }

      /* Check if any parent key columns are being modified. */
      for(p=sqlite3FkReferences(pTab); p; p=p->pNextTo){
        if( fkParentIsModified(pTab, p, aChange, chngRowid) ){
          if( p->aAction[1]!=OE_None ) return 2;
          bHaveFK = 1;
        }
      }
    }
  }
  return bHaveFK ? eRet : 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" 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].zCnName);
      sqlite3TokenInit(&tFromCol, pFKey->pFrom->aCol[iFromCol].zCnName);

      /* 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)),
          sqlite3ExprAlloc(db, TK_ID, &tFromCol, 0)
      );
      pWhere = sqlite3ExprAnd(pParse, 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)),
            sqlite3PExpr(pParse, TK_DOT,
              sqlite3ExprAlloc(db, TK_ID, &tNew, 0),
              sqlite3ExprAlloc(db, TK_ID, &tToCol, 0))
            );
        pWhen = sqlite3ExprAnd(pParse, 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));
        }else if( action==OE_SetDflt ){
          Column *pCol = pFKey->pFrom->aCol + iFromCol;
          Expr *pDflt;
          if( pCol->colFlags & COLFLAG_GENERATED ){
            testcase( pCol->colFlags & COLFLAG_VIRTUAL );
            testcase( pCol->colFlags & COLFLAG_STORED );
            pDflt = 0;
          }else{
            pDflt = sqlite3ColumnExpr(pFKey->pFrom, pCol);
          }
          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 ){
      int iDb = sqlite3SchemaToIndex(db, pTab->pSchema);
      Token tFrom;
      Token tDb;
      Expr *pRaise;

      tFrom.z = zFrom;
      tFrom.n = nFrom;
      tDb.z = db->aDb[iDb].zDbSName;
      tDb.n = sqlite3Strlen30(tDb.z);

      pRaise = sqlite3Expr(db, TK_RAISE, "FOREIGN KEY constraint failed");
      if( pRaise ){
        pRaise->affExpr = OE_Abort;
      }
      pSelect = sqlite3SelectNew(pParse,
          sqlite3ExprListAppend(pParse, 0, pRaise),
          sqlite3SrcListAppend(pParse, 0, &tDb, &tFrom),
          pWhere,
          0, 0, 0, 0, 0
      );
      pWhere = 0;
    }

    /* Disable lookaside memory allocation */
    DisableLookaside;

    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);
        pTrigger->pWhen = sqlite3ExprDup(db, pWhen, EXPRDUP_REDUCE);
      }
    }

    /* Re-enable the lookaside buffer, if it was disabled earlier. */
    EnableLookaside;

    sqlite3ExprDelete(db, pWhere);
    sqlite3ExprDelete(db, pWhen);
    sqlite3ExprListDelete(db, pList);
    sqlite3SelectDelete(db, pSelect);
    if( db->mallocFailed==1 ){
      fkTriggerDelete(db, pTrigger);
      return 0;
    }
    assert( pStep!=0 );
    assert( pTrigger!=0 );

    switch( action ){
      case OE_Restrict:
        pStep->op = TK_SELECT;
        break;
      case OE_Cascade:
        if( !pChanges ){
          pStep->op = TK_DELETE;
          break;
        }
        /* no break */ deliberate_fall_through
      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( IsOrdinaryTable(pTab) );
  assert( db!=0 );
  for(pFKey=pTab->u.tab.pFKey; pFKey; pFKey=pNext){
    assert( db==0 || sqlite3SchemaMutexHeld(db, 0, pTab->pSchema) );

    /* Remove the FK from the fkeyHash hash table. */
    if( 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 */