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Overview
Comment:Performance improvements: Avoid unnecessary seeks when doing a single-row UPDATE on a WITHOUT ROWID table.
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Files: files | file ages | folders
SHA1:6f187a0fb1b09ebc4732c4afbf3c813f82e069f1
User & Date: drh 2013-11-08 15:19:46
Context
2013-11-08
16:54
Performance improvement: Avoid unnecessary seeks on REPLACE INTO for a WITHOUT ROWID table. check-in: fd11afa5 user: drh tags: trunk
15:19
Performance improvements: Avoid unnecessary seeks when doing a single-row UPDATE on a WITHOUT ROWID table. check-in: 6f187a0f user: drh tags: trunk
12:14
Merge change to drop the mutex on the multiplexor before entering the xRead VFS call, in order to enhance parallelizability. check-in: 3c566e41 user: drh tags: trunk
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to src/sqliteInt.h.

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WhereInfo *sqlite3WhereBegin(Parse*,SrcList*,Expr*,ExprList*,ExprList*,u16,int);
void sqlite3WhereEnd(WhereInfo*);
u64 sqlite3WhereOutputRowCount(WhereInfo*);
int sqlite3WhereIsDistinct(WhereInfo*);
int sqlite3WhereIsOrdered(WhereInfo*);
int sqlite3WhereContinueLabel(WhereInfo*);
int sqlite3WhereBreakLabel(WhereInfo*);
int sqlite3WhereOkOnePass(WhereInfo*);
int sqlite3ExprCodeGetColumn(Parse*, Table*, int, int, int, u8);
void sqlite3ExprCodeGetColumnOfTable(Vdbe*, Table*, int, int, int);
void sqlite3ExprCodeMove(Parse*, int, int, int);
void sqlite3ExprCacheStore(Parse*, int, int, int);
void sqlite3ExprCachePush(Parse*);
void sqlite3ExprCachePop(Parse*, int);
void sqlite3ExprCacheRemove(Parse*, int, int);







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WhereInfo *sqlite3WhereBegin(Parse*,SrcList*,Expr*,ExprList*,ExprList*,u16,int);
void sqlite3WhereEnd(WhereInfo*);
u64 sqlite3WhereOutputRowCount(WhereInfo*);
int sqlite3WhereIsDistinct(WhereInfo*);
int sqlite3WhereIsOrdered(WhereInfo*);
int sqlite3WhereContinueLabel(WhereInfo*);
int sqlite3WhereBreakLabel(WhereInfo*);
int sqlite3WhereOkOnePass(WhereInfo*, int*);
int sqlite3ExprCodeGetColumn(Parse*, Table*, int, int, int, u8);
void sqlite3ExprCodeGetColumnOfTable(Vdbe*, Table*, int, int, int);
void sqlite3ExprCodeMove(Parse*, int, int, int);
void sqlite3ExprCacheStore(Parse*, int, int, int);
void sqlite3ExprCachePush(Parse*);
void sqlite3ExprCachePop(Parse*, int);
void sqlite3ExprCacheRemove(Parse*, int, int);

Changes to src/update.c.

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  int isView;            /* True when updating a view (INSTEAD OF trigger) */
  Trigger *pTrigger;     /* List of triggers on pTab, if required */
  int tmask;             /* Mask of TRIGGER_BEFORE|TRIGGER_AFTER */
#endif
  int newmask;           /* Mask of NEW.* columns accessed by BEFORE triggers */
  int iEph = 0;          /* Ephemeral table holding all primary key values */
  int nKey = 0;          /* Number of elements in regKey for WITHOUT ROWID */


  /* Register Allocations */
  int regRowCount = 0;   /* A count of rows changed */
  int regOldRowid;       /* The old rowid */
  int regNewRowid;       /* The new rowid */
  int regNew;            /* Content of the NEW.* table in triggers */
  int regOld = 0;        /* Content of OLD.* table in triggers */
................................................................................
  }

  /* Begin the database scan
  */
  if( HasRowid(pTab) ){
    sqlite3VdbeAddOp3(v, OP_Null, 0, regRowSet, regOldRowid);
    pWInfo = sqlite3WhereBegin(
        pParse, pTabList, pWhere, 0, 0, WHERE_ONEPASS_DESIRED, 0
    );
    if( pWInfo==0 ) goto update_cleanup;
    okOnePass = sqlite3WhereOkOnePass(pWInfo);
  
    /* Remember the rowid of every item to be updated.
    */
    sqlite3VdbeAddOp2(v, OP_Rowid, iDataCur, regOldRowid);
    if( !okOnePass ){
      sqlite3VdbeAddOp2(v, OP_RowSetAdd, regRowSet, regOldRowid);
    }
................................................................................
    pParse->nMem += nPk;
    regKey = ++pParse->nMem;
    iEph = pParse->nTab++;
    sqlite3VdbeAddOp2(v, OP_Null, 0, iPk);
    addrOpen = sqlite3VdbeAddOp2(v, OP_OpenEphemeral, iEph, nPk);
    sqlite3VdbeSetP4KeyInfo(pParse, pPk);
    pWInfo = sqlite3WhereBegin(pParse, pTabList, pWhere, 0, 0, 
                               WHERE_ONEPASS_DESIRED, 0);
    if( pWInfo==0 ) goto update_cleanup;
    okOnePass = sqlite3WhereOkOnePass(pWInfo);
    for(i=0; i<nPk; i++){
      sqlite3ExprCodeGetColumnOfTable(v, pTab, iDataCur, pPk->aiColumn[i],
                                      iPk+i);
    }
    if( okOnePass ){
      sqlite3VdbeChangeToNoop(v, addrOpen);
      nKey = nPk;
................................................................................
  /* Initialize the count of updated rows
  */
  if( (db->flags & SQLITE_CountRows) && !pParse->pTriggerTab ){
    regRowCount = ++pParse->nMem;
    sqlite3VdbeAddOp2(v, OP_Integer, 0, regRowCount);
  }


  if( !isView ){
    /* 
    ** Open every index that needs updating.  Note that if any
    ** index could potentially invoke a REPLACE conflict resolution 
    ** action, then we need to open all indices because we might need
    ** to be deleting some records.
    */
................................................................................
        if( pIdx->onError==OE_Replace ){
          openAll = 1;
          break;
        }
      }
    }
    for(i=0, pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext, i++){

      assert( aRegIdx );
      if( openAll || aRegIdx[i]>0 ){



        sqlite3VdbeAddOp3(v, OP_OpenWrite, iIdxCur+i, pIdx->tnum, iDb);
        sqlite3VdbeSetP4KeyInfo(pParse, pIdx);
        assert( pParse->nTab>iIdxCur+i );
        VdbeComment((v, "%s", pIdx->zName));




      }
    }
  }

  /* Top of the update loop */
  labelBreak = sqlite3VdbeMakeLabel(v);
  if( okOnePass ){
    labelContinue = labelBreak;
    sqlite3VdbeAddOp2(v, OP_IsNull, pPk ? regKey : regOldRowid, labelBreak);
    if( pPk ){
      sqlite3VdbeAddOp4Int(v, OP_NotFound, iDataCur, labelBreak, regKey, nKey);
    }
  }else if( pPk ){
    labelContinue = sqlite3VdbeMakeLabel(v);
    sqlite3VdbeAddOp2(v, OP_Rewind, iEph, labelBreak);
    addrTop = sqlite3VdbeAddOp2(v, OP_RowKey, iEph, regKey);
    sqlite3VdbeAddOp4Int(v, OP_NotFound, iDataCur, labelContinue, regKey, 0);
  }else{
    labelContinue = sqlite3VdbeAddOp3(v, OP_RowSetRead, regRowSet, labelBreak,
................................................................................
  }
  sqlite3VdbeResolveLabel(v, labelBreak);

  /* Close all tables */
  for(i=0, pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext, i++){
    assert( aRegIdx );
    if( openAll || aRegIdx[i]>0 ){
      sqlite3VdbeAddOp2(v, OP_Close, iIdxCur, 0);
    }
  }
  if( iDataCur<iIdxCur ) sqlite3VdbeAddOp2(v, OP_Close, iDataCur, 0);

  /* Update the sqlite_sequence table by storing the content of the
  ** maximum rowid counter values recorded while inserting into
  ** autoincrement tables.







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  int isView;            /* True when updating a view (INSTEAD OF trigger) */
  Trigger *pTrigger;     /* List of triggers on pTab, if required */
  int tmask;             /* Mask of TRIGGER_BEFORE|TRIGGER_AFTER */
#endif
  int newmask;           /* Mask of NEW.* columns accessed by BEFORE triggers */
  int iEph = 0;          /* Ephemeral table holding all primary key values */
  int nKey = 0;          /* Number of elements in regKey for WITHOUT ROWID */
  int aiCurOnePass[2];   /* The write cursors opened by WHERE_ONEPASS */

  /* Register Allocations */
  int regRowCount = 0;   /* A count of rows changed */
  int regOldRowid;       /* The old rowid */
  int regNewRowid;       /* The new rowid */
  int regNew;            /* Content of the NEW.* table in triggers */
  int regOld = 0;        /* Content of OLD.* table in triggers */
................................................................................
  }

  /* Begin the database scan
  */
  if( HasRowid(pTab) ){
    sqlite3VdbeAddOp3(v, OP_Null, 0, regRowSet, regOldRowid);
    pWInfo = sqlite3WhereBegin(
        pParse, pTabList, pWhere, 0, 0, WHERE_ONEPASS_DESIRED, iIdxCur
    );
    if( pWInfo==0 ) goto update_cleanup;
    okOnePass = sqlite3WhereOkOnePass(pWInfo, aiCurOnePass);
  
    /* Remember the rowid of every item to be updated.
    */
    sqlite3VdbeAddOp2(v, OP_Rowid, iDataCur, regOldRowid);
    if( !okOnePass ){
      sqlite3VdbeAddOp2(v, OP_RowSetAdd, regRowSet, regOldRowid);
    }
................................................................................
    pParse->nMem += nPk;
    regKey = ++pParse->nMem;
    iEph = pParse->nTab++;
    sqlite3VdbeAddOp2(v, OP_Null, 0, iPk);
    addrOpen = sqlite3VdbeAddOp2(v, OP_OpenEphemeral, iEph, nPk);
    sqlite3VdbeSetP4KeyInfo(pParse, pPk);
    pWInfo = sqlite3WhereBegin(pParse, pTabList, pWhere, 0, 0, 
                               WHERE_ONEPASS_DESIRED, iIdxCur);
    if( pWInfo==0 ) goto update_cleanup;
    okOnePass = sqlite3WhereOkOnePass(pWInfo, aiCurOnePass);
    for(i=0; i<nPk; i++){
      sqlite3ExprCodeGetColumnOfTable(v, pTab, iDataCur, pPk->aiColumn[i],
                                      iPk+i);
    }
    if( okOnePass ){
      sqlite3VdbeChangeToNoop(v, addrOpen);
      nKey = nPk;
................................................................................
  /* Initialize the count of updated rows
  */
  if( (db->flags & SQLITE_CountRows) && !pParse->pTriggerTab ){
    regRowCount = ++pParse->nMem;
    sqlite3VdbeAddOp2(v, OP_Integer, 0, regRowCount);
  }

  labelBreak = sqlite3VdbeMakeLabel(v);
  if( !isView ){
    /* 
    ** Open every index that needs updating.  Note that if any
    ** index could potentially invoke a REPLACE conflict resolution 
    ** action, then we need to open all indices because we might need
    ** to be deleting some records.
    */
................................................................................
        if( pIdx->onError==OE_Replace ){
          openAll = 1;
          break;
        }
      }
    }
    for(i=0, pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext, i++){
      int iThisCur = iIdxCur+i;
      assert( aRegIdx );
      if( (openAll || aRegIdx[i]>0)
       && iThisCur!=aiCurOnePass[0]
       && iThisCur!=aiCurOnePass[1]
      ){
        sqlite3VdbeAddOp3(v, OP_OpenWrite, iThisCur, pIdx->tnum, iDb);
        sqlite3VdbeSetP4KeyInfo(pParse, pIdx);
        assert( pParse->nTab>iThisCur );
        VdbeComment((v, "%s", pIdx->zName));
        if( okOnePass && pPk && iThisCur==iDataCur ){
          sqlite3VdbeAddOp4Int(v, OP_NotFound, iDataCur, labelBreak,
                               regKey, nKey);
        }
      }
    }
  }

  /* Top of the update loop */

  if( okOnePass ){
    labelContinue = labelBreak;
    sqlite3VdbeAddOp2(v, OP_IsNull, pPk ? regKey : regOldRowid, labelBreak);



  }else if( pPk ){
    labelContinue = sqlite3VdbeMakeLabel(v);
    sqlite3VdbeAddOp2(v, OP_Rewind, iEph, labelBreak);
    addrTop = sqlite3VdbeAddOp2(v, OP_RowKey, iEph, regKey);
    sqlite3VdbeAddOp4Int(v, OP_NotFound, iDataCur, labelContinue, regKey, 0);
  }else{
    labelContinue = sqlite3VdbeAddOp3(v, OP_RowSetRead, regRowSet, labelBreak,
................................................................................
  }
  sqlite3VdbeResolveLabel(v, labelBreak);

  /* Close all tables */
  for(i=0, pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext, i++){
    assert( aRegIdx );
    if( openAll || aRegIdx[i]>0 ){
      sqlite3VdbeAddOp2(v, OP_Close, iIdxCur+i, 0);
    }
  }
  if( iDataCur<iIdxCur ) sqlite3VdbeAddOp2(v, OP_Close, iDataCur, 0);

  /* Update the sqlite_sequence table by storing the content of the
  ** maximum rowid counter values recorded while inserting into
  ** autoincrement tables.

Changes to src/vdbe.c.

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  if( c ){
    pc = pOp->p2-1;
  }
  break;
}

/* Opcode: IsNull P1 P2 * * *

**
** Jump to P2 if the value in register P1 is NULL.
*/
case OP_IsNull: {            /* same as TK_ISNULL, jump, in1 */
  pIn1 = &aMem[pOp->p1];
  if( (pIn1->flags & MEM_Null)!=0 ){
    pc = pOp->p2 - 1;
  }
  break;
}

/* Opcode: NotNull P1 P2 * * *

**
** Jump to P2 if the value in register P1 is not NULL.  
*/
case OP_NotNull: {            /* same as TK_NOTNULL, jump, in1 */
  pIn1 = &aMem[pOp->p1];
  if( (pIn1->flags & MEM_Null)==0 ){
    pc = pOp->p2 - 1;







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  if( c ){
    pc = pOp->p2-1;
  }
  break;
}

/* Opcode: IsNull P1 P2 * * *
** Synopsis:  if r[P1]==NULL goto P2
**
** Jump to P2 if the value in register P1 is NULL.
*/
case OP_IsNull: {            /* same as TK_ISNULL, jump, in1 */
  pIn1 = &aMem[pOp->p1];
  if( (pIn1->flags & MEM_Null)!=0 ){
    pc = pOp->p2 - 1;
  }
  break;
}

/* Opcode: NotNull P1 P2 * * *
** Synopsis: if r[P1]!=NULL goto P2
**
** Jump to P2 if the value in register P1 is not NULL.  
*/
case OP_NotNull: {            /* same as TK_NOTNULL, jump, in1 */
  pIn1 = &aMem[pOp->p1];
  if( (pIn1->flags & MEM_Null)==0 ){
    pc = pOp->p2 - 1;

Changes to src/where.c.

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  u8 untestedTerms;         /* Not all WHERE terms resolved by outer loop */
  u8 eDistinct;             /* One of the WHERE_DISTINCT_* values below */
  u8 nLevel;                /* Number of nested loop */
  int iTop;                 /* The very beginning of the WHERE loop */
  int iContinue;            /* Jump here to continue with next record */
  int iBreak;               /* Jump here to break out of the loop */
  int savedNQueryLoop;      /* pParse->nQueryLoop outside the WHERE loop */

  WhereMaskSet sMaskSet;    /* Map cursor numbers to bitmasks */
  WhereClause sWC;          /* Decomposition of the WHERE clause */
  WhereLevel a[1];          /* Information about each nest loop in WHERE */
};

/*
** Bitmasks for the operators on WhereTerm objects.  These are all
................................................................................
  return pWInfo->iBreak;
}

/*
** Return TRUE if an UPDATE or DELETE statement can operate directly on
** the rowids returned by a WHERE clause.  Return FALSE if doing an
** UPDATE or DELETE might change subsequent WHERE clause results.










*/
int sqlite3WhereOkOnePass(WhereInfo *pWInfo){

  return pWInfo->okOnePass;
}

/*
** Move the content of pSrc into pDest
*/
static void whereOrMove(WhereOrSet *pDest, WhereOrSet *pSrc){
................................................................................
  pLoop = pLevel->pWLoop;
  pTabItem = &pWInfo->pTabList->a[pLevel->iFrom];
  iCur = pTabItem->iCursor;
  pLevel->notReady = notReady & ~getMask(&pWInfo->sMaskSet, iCur);
  bRev = (pWInfo->revMask>>iLevel)&1;
  omitTable = (pLoop->wsFlags & WHERE_IDX_ONLY)!=0 
           && (pWInfo->wctrlFlags & WHERE_FORCE_TABLE)==0;
  VdbeNoopComment((v, "Begin Join Loop %d", iLevel));

  /* Create labels for the "break" and "continue" instructions
  ** for the current loop.  Jump to addrBrk to break out of a loop.
  ** Jump to cont to go immediately to the next iteration of the
  ** loop.
  **
  ** When there is an IN operator, we also have a "addrNxt" label that
................................................................................
**
** ORDER BY CLAUSE PROCESSING
**
** pOrderBy is a pointer to the ORDER BY clause (or the GROUP BY clause
** if the WHERE_GROUPBY flag is set in wctrlFlags) of a SELECT statement
** if there is one.  If there is no ORDER BY clause or if this routine
** is called from an UPDATE or DELETE statement, then pOrderBy is NULL.








*/
WhereInfo *sqlite3WhereBegin(
  Parse *pParse,        /* The parser context */
  SrcList *pTabList,    /* FROM clause: A list of all tables to be scanned */
  Expr *pWhere,         /* The WHERE clause */
  ExprList *pOrderBy,   /* An ORDER BY clause, or NULL */
  ExprList *pResultSet, /* Result set of the query */
................................................................................
  nByteWInfo = ROUND8(sizeof(WhereInfo)+(nTabList-1)*sizeof(WhereLevel));
  pWInfo = sqlite3DbMallocZero(db, nByteWInfo + sizeof(WhereLoop));
  if( db->mallocFailed ){
    sqlite3DbFree(db, pWInfo);
    pWInfo = 0;
    goto whereBeginError;
  }

  pWInfo->nLevel = nTabList;
  pWInfo->pParse = pParse;
  pWInfo->pTabList = pTabList;
  pWInfo->pOrderBy = pOrderBy;
  pWInfo->pResultSet = pResultSet;
  pWInfo->iBreak = sqlite3VdbeMakeLabel(v);
  pWInfo->wctrlFlags = wctrlFlags;
................................................................................
      sqlite3VdbeAddOp4(v, OP_VOpen, iCur, 0, 0, pVTab, P4_VTAB);
    }else if( IsVirtual(pTab) ){
      /* noop */
    }else
#endif
    if( (pLoop->wsFlags & WHERE_IDX_ONLY)==0
         && (wctrlFlags & WHERE_OMIT_OPEN_CLOSE)==0 ){

      int op = pWInfo->okOnePass ? OP_OpenWrite : OP_OpenRead;



      sqlite3OpenTable(pParse, pTabItem->iCursor, iDb, pTab, op);

      testcase( !pWInfo->okOnePass && pTab->nCol==BMS-1 );
      testcase( !pWInfo->okOnePass && pTab->nCol==BMS );
      if( !pWInfo->okOnePass && pTab->nCol<BMS && HasRowid(pTab) ){
        Bitmask b = pTabItem->colUsed;
        int n = 0;
        for(; b; b=b>>1, n++){}
        sqlite3VdbeChangeP4(v, sqlite3VdbeCurrentAddr(v)-1, 
................................................................................
        assert( n<=pTab->nCol );
      }
    }else{
      sqlite3TableLock(pParse, iDb, pTab->tnum, 0, pTab->zName);
    }
    if( pLoop->wsFlags & WHERE_INDEXED ){
      Index *pIx = pLoop->u.btree.pIndex;
      /* FIXME:  As an optimization use pTabItem->iCursor if WHERE_IDX_ONLY */
















      int iIndexCur = pLevel->iIdxCur = iIdxCur ? iIdxCur : pParse->nTab++;
      assert( pIx->pSchema==pTab->pSchema );
      assert( iIndexCur>=0 );
      sqlite3VdbeAddOp3(v, OP_OpenRead, iIndexCur, pIx->tnum, iDb);
      sqlite3VdbeSetP4KeyInfo(pParse, pIx);
      VdbeComment((v, "%s", pIx->zName));
    }
    sqlite3CodeVerifySchema(pParse, iDb);
    notReady &= ~getMask(&pWInfo->sMaskSet, pTabItem->iCursor);
  }
  pWInfo->iTop = sqlite3VdbeCurrentAddr(v);
................................................................................
      if( pLevel->op==OP_Return ){
        sqlite3VdbeAddOp2(v, OP_Gosub, pLevel->p1, pLevel->addrFirst);
      }else{
        sqlite3VdbeAddOp2(v, OP_Goto, 0, pLevel->addrFirst);
      }
      sqlite3VdbeJumpHere(v, addr);
    }


  }

  /* The "break" point is here, just past the end of the outer loop.
  ** Set it.
  */
  sqlite3VdbeResolveLabel(v, pWInfo->iBreak);

  /* Close all of the cursors that were opened by sqlite3WhereBegin.
  */
  assert( pWInfo->nLevel<=pTabList->nSrc );
  for(i=0, pLevel=pWInfo->a; i<pWInfo->nLevel; i++, pLevel++){
    Index *pIdx = 0;
    struct SrcList_item *pTabItem = &pTabList->a[pLevel->iFrom];
    Table *pTab = pTabItem->pTab;
    assert( pTab!=0 );
    pLoop = pLevel->pWLoop;






    if( (pTab->tabFlags & TF_Ephemeral)==0
     && pTab->pSelect==0
     && (pWInfo->wctrlFlags & WHERE_OMIT_OPEN_CLOSE)==0
    ){
      int ws = pLoop->wsFlags;
      if( !pWInfo->okOnePass && (ws & WHERE_IDX_ONLY)==0 ){
        sqlite3VdbeAddOp1(v, OP_Close, pTabItem->iCursor);
      }

      if( (ws & WHERE_INDEXED)!=0 && (ws & (WHERE_IPK|WHERE_AUTO_INDEX))==0 ){


        sqlite3VdbeAddOp1(v, OP_Close, pLevel->iIdxCur);
      }
    }

    /* If this scan uses an index, make VDBE code substitutions to read data
    ** from the index instead of from the table where possible.  In some cases
    ** this optimization prevents the table from ever being read, which can







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...
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6224
  u8 untestedTerms;         /* Not all WHERE terms resolved by outer loop */
  u8 eDistinct;             /* One of the WHERE_DISTINCT_* values below */
  u8 nLevel;                /* Number of nested loop */
  int iTop;                 /* The very beginning of the WHERE loop */
  int iContinue;            /* Jump here to continue with next record */
  int iBreak;               /* Jump here to break out of the loop */
  int savedNQueryLoop;      /* pParse->nQueryLoop outside the WHERE loop */
  int aiCurOnePass[2];      /* OP_OpenWrite cursors for the ONEPASS opt */
  WhereMaskSet sMaskSet;    /* Map cursor numbers to bitmasks */
  WhereClause sWC;          /* Decomposition of the WHERE clause */
  WhereLevel a[1];          /* Information about each nest loop in WHERE */
};

/*
** Bitmasks for the operators on WhereTerm objects.  These are all
................................................................................
  return pWInfo->iBreak;
}

/*
** Return TRUE if an UPDATE or DELETE statement can operate directly on
** the rowids returned by a WHERE clause.  Return FALSE if doing an
** UPDATE or DELETE might change subsequent WHERE clause results.
**
** If the ONEPASS optimization is used (if this routine returns true)
** then also write the indices of open cursors used by ONEPASS
** into aiCur[0] and aiCur[1].  iaCur[0] gets the cursor of the data
** table and iaCur[1] gets the cursor used by an auxiliary index.
** Either value may be -1, indicating that cursor is not used.
** Any cursors returned will have been opened for writing.
**
** aiCur[0] and aiCur[1] both get -1 if the where-clause logic is
** unable to use the ONEPASS optimization.
*/
int sqlite3WhereOkOnePass(WhereInfo *pWInfo, int *aiCur){
  memcpy(aiCur, pWInfo->aiCurOnePass, sizeof(int)*2);
  return pWInfo->okOnePass;
}

/*
** Move the content of pSrc into pDest
*/
static void whereOrMove(WhereOrSet *pDest, WhereOrSet *pSrc){
................................................................................
  pLoop = pLevel->pWLoop;
  pTabItem = &pWInfo->pTabList->a[pLevel->iFrom];
  iCur = pTabItem->iCursor;
  pLevel->notReady = notReady & ~getMask(&pWInfo->sMaskSet, iCur);
  bRev = (pWInfo->revMask>>iLevel)&1;
  omitTable = (pLoop->wsFlags & WHERE_IDX_ONLY)!=0 
           && (pWInfo->wctrlFlags & WHERE_FORCE_TABLE)==0;
  VdbeNoopComment((v, "Begin WHERE-Loop %d: %s", iLevel,pTabItem->pTab->zName));

  /* Create labels for the "break" and "continue" instructions
  ** for the current loop.  Jump to addrBrk to break out of a loop.
  ** Jump to cont to go immediately to the next iteration of the
  ** loop.
  **
  ** When there is an IN operator, we also have a "addrNxt" label that
................................................................................
**
** ORDER BY CLAUSE PROCESSING
**
** pOrderBy is a pointer to the ORDER BY clause (or the GROUP BY clause
** if the WHERE_GROUPBY flag is set in wctrlFlags) of a SELECT statement
** if there is one.  If there is no ORDER BY clause or if this routine
** is called from an UPDATE or DELETE statement, then pOrderBy is NULL.
**
** The iIdxCur parameter is the cursor number of an index.  If 
** WHERE_ONETABLE_ONLY is set, iIdxCur is the cursor number of an index
** to use for OR clause processing.  The WHERE clause should use this
** specific cursor.  If WHERE_ONEPASS_DESIRED is set, then iIdxCur is
** the first cursor in an array of cursors for all indices.  iIdxCur should
** be used to compute the appropriate cursor depending on which index is
** used.
*/
WhereInfo *sqlite3WhereBegin(
  Parse *pParse,        /* The parser context */
  SrcList *pTabList,    /* FROM clause: A list of all tables to be scanned */
  Expr *pWhere,         /* The WHERE clause */
  ExprList *pOrderBy,   /* An ORDER BY clause, or NULL */
  ExprList *pResultSet, /* Result set of the query */
................................................................................
  nByteWInfo = ROUND8(sizeof(WhereInfo)+(nTabList-1)*sizeof(WhereLevel));
  pWInfo = sqlite3DbMallocZero(db, nByteWInfo + sizeof(WhereLoop));
  if( db->mallocFailed ){
    sqlite3DbFree(db, pWInfo);
    pWInfo = 0;
    goto whereBeginError;
  }
  pWInfo->aiCurOnePass[0] = pWInfo->aiCurOnePass[1] = -1;
  pWInfo->nLevel = nTabList;
  pWInfo->pParse = pParse;
  pWInfo->pTabList = pTabList;
  pWInfo->pOrderBy = pOrderBy;
  pWInfo->pResultSet = pResultSet;
  pWInfo->iBreak = sqlite3VdbeMakeLabel(v);
  pWInfo->wctrlFlags = wctrlFlags;
................................................................................
      sqlite3VdbeAddOp4(v, OP_VOpen, iCur, 0, 0, pVTab, P4_VTAB);
    }else if( IsVirtual(pTab) ){
      /* noop */
    }else
#endif
    if( (pLoop->wsFlags & WHERE_IDX_ONLY)==0
         && (wctrlFlags & WHERE_OMIT_OPEN_CLOSE)==0 ){
      int op = OP_OpenRead;
      if( pWInfo->okOnePass ){
        op = OP_OpenWrite;
        pWInfo->aiCurOnePass[0] = pTabItem->iCursor;
      };
      sqlite3OpenTable(pParse, pTabItem->iCursor, iDb, pTab, op);
      assert( pTabItem->iCursor==pLevel->iTabCur );
      testcase( !pWInfo->okOnePass && pTab->nCol==BMS-1 );
      testcase( !pWInfo->okOnePass && pTab->nCol==BMS );
      if( !pWInfo->okOnePass && pTab->nCol<BMS && HasRowid(pTab) ){
        Bitmask b = pTabItem->colUsed;
        int n = 0;
        for(; b; b=b>>1, n++){}
        sqlite3VdbeChangeP4(v, sqlite3VdbeCurrentAddr(v)-1, 
................................................................................
        assert( n<=pTab->nCol );
      }
    }else{
      sqlite3TableLock(pParse, iDb, pTab->tnum, 0, pTab->zName);
    }
    if( pLoop->wsFlags & WHERE_INDEXED ){
      Index *pIx = pLoop->u.btree.pIndex;
      int iIndexCur;
      int op = OP_OpenRead;
      if( pWInfo->okOnePass && iIdxCur ){
        Index *pJ = pTabItem->pTab->pIndex;
        iIndexCur = iIdxCur;
        assert( wctrlFlags & WHERE_ONEPASS_DESIRED );
        while( ALWAYS(pJ) && pJ!=pIx ){
          iIndexCur++;
          pJ = pJ->pNext;
        }
        op = OP_OpenWrite;
        pWInfo->aiCurOnePass[1] = iIndexCur;
      }else if( iIdxCur && (wctrlFlags & WHERE_ONETABLE_ONLY)!=0 ){
        iIndexCur = iIdxCur;
      }else{
        iIndexCur = pParse->nTab++;
      }
      pLevel->iIdxCur = iIndexCur;
      assert( pIx->pSchema==pTab->pSchema );
      assert( iIndexCur>=0 );
      sqlite3VdbeAddOp3(v, op, iIndexCur, pIx->tnum, iDb);
      sqlite3VdbeSetP4KeyInfo(pParse, pIx);
      VdbeComment((v, "%s", pIx->zName));
    }
    sqlite3CodeVerifySchema(pParse, iDb);
    notReady &= ~getMask(&pWInfo->sMaskSet, pTabItem->iCursor);
  }
  pWInfo->iTop = sqlite3VdbeCurrentAddr(v);
................................................................................
      if( pLevel->op==OP_Return ){
        sqlite3VdbeAddOp2(v, OP_Gosub, pLevel->p1, pLevel->addrFirst);
      }else{
        sqlite3VdbeAddOp2(v, OP_Goto, 0, pLevel->addrFirst);
      }
      sqlite3VdbeJumpHere(v, addr);
    }
    VdbeNoopComment((v, "End WHERE-Loop %d: %s", i,
                     pWInfo->pTabList->a[pLevel->iFrom].pTab->zName));
  }

  /* The "break" point is here, just past the end of the outer loop.
  ** Set it.
  */
  sqlite3VdbeResolveLabel(v, pWInfo->iBreak);



  assert( pWInfo->nLevel<=pTabList->nSrc );
  for(i=0, pLevel=pWInfo->a; i<pWInfo->nLevel; i++, pLevel++){
    Index *pIdx = 0;
    struct SrcList_item *pTabItem = &pTabList->a[pLevel->iFrom];
    Table *pTab = pTabItem->pTab;
    assert( pTab!=0 );
    pLoop = pLevel->pWLoop;

    /* Close all of the cursors that were opened by sqlite3WhereBegin.
    ** Except, do not close cursors that will be reused by the OR optimization
    ** (WHERE_OMIT_OPEN_CLOSE).  And do not close the OP_OpenWrite cursors
    ** created for the ONEPASS optimization.
    */
    if( (pTab->tabFlags & TF_Ephemeral)==0
     && pTab->pSelect==0
     && (pWInfo->wctrlFlags & WHERE_OMIT_OPEN_CLOSE)==0
    ){
      int ws = pLoop->wsFlags;
      if( !pWInfo->okOnePass && (ws & WHERE_IDX_ONLY)==0 ){
        sqlite3VdbeAddOp1(v, OP_Close, pTabItem->iCursor);
      }
      if( (ws & WHERE_INDEXED)!=0
       && (ws & (WHERE_IPK|WHERE_AUTO_INDEX))==0 
       && pLevel->iIdxCur!=pWInfo->aiCurOnePass[1]
      ){
        sqlite3VdbeAddOp1(v, OP_Close, pLevel->iIdxCur);
      }
    }

    /* If this scan uses an index, make VDBE code substitutions to read data
    ** from the index instead of from the table where possible.  In some cases
    ** this optimization prevents the table from ever being read, which can