SQLite

  u8 eCode;                                 /* A small processing code */
  union {                                   /* Extra data for callback */
    NameContext *pNC;                          /* Naming context */
    int n;                                     /* A counter */
    int iCur;                                  /* A cursor number */
    SrcList *pSrcList;                         /* FROM clause */
    struct SrcCount *pSrcCount;                /* Counting column references */
    struct CCurHint *pCCurHint;                /* Used by codeCursorHint() */
  } u;
};

/* Forward declarations */
int sqlite3WalkExpr(Walker*, Expr*);
int sqlite3WalkExprList(Walker*, ExprList*);
int sqlite3WalkSelect(Walker*, Select*);

    pMem->u.i = pOp->p2;                          /* P2 */
    pMem++;

    pMem->flags = MEM_Int;
    pMem->u.i = pOp->p3;                          /* P3 */
    pMem++;

    if( sqlite3VdbeMemClearAndResize(pMem, 100) ){ /* P4 */
      assert( p->db->mallocFailed );
      return SQLITE_ERROR;
    }
    pMem->flags = MEM_Str|MEM_Term;
    zP4 = displayP4(pOp, pMem->z, pMem->szMalloc);
    if( zP4!=pMem->z ){
      sqlite3VdbeMemSetStr(pMem, zP4, -1, SQLITE_UTF8, 0);
    }else{
      assert( pMem->z!=0 );
      pMem->n = sqlite3Strlen30(pMem->z);
      pMem->enc = SQLITE_UTF8;
    }

        Bitmask b = pTabItem->colUsed;
        int n = 0;
        for(; b; b=b>>1, n++){}
        sqlite3VdbeChangeP4(v, sqlite3VdbeCurrentAddr(v)-1, 
                            SQLITE_INT_TO_PTR(n), P4_INT32);
        assert( n<=pTab->nCol );
      }
#ifdef SQLITE_ENABLE_CURSOR_HINTS
      if( pLoop->u.btree.pIndex!=0 ) sqlite3VdbeChangeP5(v, OPFLAG_SEEKEQ);
#endif
#ifdef SQLITE_ENABLE_COLUMN_USED_MASK
      sqlite3VdbeAddOp4Dup8(v, OP_ColumnsUsed, pTabItem->iCursor, 0, 0,
                            (const u8*)&pTabItem->colUsed, P4_INT64);
#endif
    }else{
      sqlite3TableLock(pParse, iDb, pTab->tnum, 0, pTab->zName);
    }

            || pTerm->pWC->pWInfo->pParse->db->mallocFailed );
    pOp->p3 = pLevel->iLikeRepCntr;
    pOp->p5 = 1;
  }
}

#ifdef SQLITE_ENABLE_CURSOR_HINTS
/*
** Information is passed from codeCursorHint() down to individual nodes of
** the expression tree (by sqlite3WalkExpr()) using an instance of this
** structure.
*/
struct CCurHint {
  int iTabCur;    /* Cursor for the main table */
  int iIdxCur;    /* Cursor for the index, if pIdx!=0.  Unused otherwise */
  Index *pIdx;    /* The index used to access the table */
};

/*
** This function is called for every node of an expression that is a candidate
** for a cursor hint on an index cursor.  For TK_COLUMN nodes that reference
** the table CCurHint.iTabCur, verify that the same column can be
** accessed through the index.  If it cannot, then set pWalker->eCode to 1.
*/
static int codeCursorHintCheckExpr(Walker *pWalker, Expr *pExpr){
  struct CCurHint *pHint = pWalker->u.pCCurHint;
  assert( pHint->pIdx!=0 );
  if( pExpr->op==TK_COLUMN
   && pExpr->iTable==pHint->iTabCur
   && sqlite3ColumnOfIndex(pHint->pIdx, pExpr->iColumn)<0
  ){
    pWalker->eCode = 1;
  }
  return WRC_Continue;
}


/*
** This function is called on every node of an expression tree used as an
** argument to the OP_CursorHint instruction. If the node is a TK_COLUMN
** that accesses any table other than the one identified by
** CCurHint.iTabCur, then do the following:
**
**   1) allocate a register and code an OP_Column instruction to read 
**      the specified column into the new register, and
**
**   2) transform the expression node to a TK_REGISTER node that reads 
**      from the newly populated register.
**
** Also, if the node is a TK_COLUMN that does access the table idenified
** by pCCurHint.iTabCur, and an index is being used (which we will
** know because CCurHint.pIdx!=0) then transform the TK_COLUMN into
** an access of the index rather than the original table.
*/
static int codeCursorHintFixExpr(Walker *pWalker, Expr *pExpr){
  int rc = WRC_Continue;
  struct CCurHint *pHint = pWalker->u.pCCurHint;
  if( pExpr->op==TK_COLUMN ){
    if( pExpr->iTable!=pHint->iTabCur ){
      Vdbe *v = pWalker->pParse->pVdbe;
      int reg = ++pWalker->pParse->nMem;   /* Register for column value */
      sqlite3ExprCodeGetColumnOfTable(
          v, pExpr->pTab, pExpr->iTable, pExpr->iColumn, reg
      );
      pExpr->op = TK_REGISTER;
      pExpr->iTable = reg;
    }else if( pHint->pIdx!=0 ){
      pExpr->iTable = pHint->iIdxCur;
      pExpr->iColumn = sqlite3ColumnOfIndex(pHint->pIdx, pExpr->iColumn);
      assert( pExpr->iColumn>=0 );
    }
  }else if( pExpr->op==TK_AGG_FUNCTION ){
    /* An aggregate function in the WHERE clause of a query means this must
    ** be a correlated sub-query, and expression pExpr is an aggregate from
    ** the parent context. Do not walk the function arguments in this case.
    **
    ** todo: It should be possible to replace this node with a TK_REGISTER
    ** expression, as the result of the expression must be stored in a 
    ** register at this point. The same holds for TK_AGG_COLUMN nodes. */
    rc = WRC_Prune;
  }
  return rc;
}

/*
** Insert an OP_CursorHint instruction if it is appropriate to do so.
*/
static void codeCursorHint(
  WhereInfo *pWInfo,
  WhereLevel *pLevel
){
  Parse *pParse = pWInfo->pParse;
  sqlite3 *db = pParse->db;
  Vdbe *v = pParse->pVdbe;

  Expr *pExpr = 0;
  WhereLoop *pLoop = pLevel->pWLoop;
  int iCur;
  WhereClause *pWC;
  WhereTerm *pTerm;
  int i;
  struct CCurHint sHint;
  Walker sWalker;

  if( OptimizationDisabled(db, SQLITE_CursorHints) ) return;
  iCur = pLevel->iTabCur;
  assert( iCur==pWInfo->pTabList->a[pLevel->iFrom].iCursor );
  sHint.iTabCur = iCur;
  sHint.iIdxCur = pLevel->iIdxCur;
  sHint.pIdx = pLoop->u.btree.pIndex;
  memset(&sWalker, 0, sizeof(sWalker));
  sWalker.pParse = pParse;
  sWalker.u.pCCurHint = &sHint;
  pWC = &pWInfo->sWC;
  for(i=0; i<pWC->nTerm; i++){
    pTerm = &pWC->a[i];
    if( pTerm->wtFlags & (TERM_VIRTUAL|TERM_CODED) ) continue;
    if( pTerm->prereqAll & pLevel->notReady ) continue;
    if( ExprHasProperty(pTerm->pExpr, EP_FromJoin) ) continue;
    if( sqlite3ExprContainsSubquery(pTerm->pExpr) ) continue;
    if( sHint.pIdx!=0 ){
      sWalker.eCode = 0;
      sWalker.xExprCallback = codeCursorHintCheckExpr;
      sqlite3WalkExpr(&sWalker, pTerm->pExpr);
      if( sWalker.eCode ) continue;
    }
    pExpr = sqlite3ExprAnd(db, pExpr, sqlite3ExprDup(db, pTerm->pExpr, 0));
  }
  if( pExpr!=0 ){



    sWalker.xExprCallback = codeCursorHintFixExpr;


    sqlite3WalkExpr(&sWalker, pExpr);
    sqlite3VdbeAddOp4(v, OP_CursorHint, 
                      (sHint.pIdx ? sHint.iIdxCur : sHint.iTabCur), 0, 0,
                      (const char*)pExpr, P4_EXPR);
  }
}
#else
# define codeCursorHint(A,B)  /* No-op */
#endif /* SQLITE_ENABLE_CURSOR_HINTS */

/*

    if( pLoop->wsFlags & WHERE_TOP_LIMIT ) pEnd = pLoop->aLTerm[j++];
    assert( pStart!=0 || pEnd!=0 );
    if( bRev ){
      pTerm = pStart;
      pStart = pEnd;
      pEnd = pTerm;
    }
    codeCursorHint(pWInfo, pLevel);
    if( pStart ){
      Expr *pX;             /* The expression that defines the start bound */
      int r1, rTemp;        /* Registers for holding the start boundary */

      /* The following constant maps TK_xx codes into corresponding 
      ** seek opcodes.  It depends on a particular ordering of TK_xx
      */

    testcase( pRangeEnd && (pRangeEnd->eOperator & WO_LE)!=0 );
    testcase( pRangeEnd && (pRangeEnd->eOperator & WO_GE)!=0 );
    startEq = !pRangeStart || pRangeStart->eOperator & (WO_LE|WO_GE);
    endEq =   !pRangeEnd || pRangeEnd->eOperator & (WO_LE|WO_GE);
    start_constraints = pRangeStart || nEq>0;

    /* Seek the index cursor to the start of the range. */
    codeCursorHint(pWInfo, pLevel);
    nConstraint = nEq;
    if( pRangeStart ){
      Expr *pRight = pRangeStart->pExpr->pRight;
      sqlite3ExprCode(pParse, pRight, regBase+nEq);
      whereLikeOptimizationStringFixup(v, pLevel, pRangeStart);
      if( (pRangeStart->wtFlags & TERM_VNULL)==0
       && sqlite3ExprCanBeNull(pRight)

    static const u8 aStart[] = { OP_Rewind, OP_Last };
    assert( bRev==0 || bRev==1 );
    if( pTabItem->isRecursive ){
      /* Tables marked isRecursive have only a single row that is stored in
      ** a pseudo-cursor.  No need to Rewind or Next such cursors. */
      pLevel->op = OP_Noop;
    }else{
      codeCursorHint(pWInfo, pLevel);
      pLevel->op = aStep[bRev];
      pLevel->p1 = iCur;
      pLevel->p2 = 1 + sqlite3VdbeAddOp2(v, aStart[bRev], iCur, addrBrk);
      VdbeCoverageIf(v, bRev==0);
      VdbeCoverageIf(v, bRev!=0);
      pLevel->p5 = SQLITE_STMTSTATUS_FULLSCAN_STEP;
    }