SQLite

   && sqlite3ExprCompare(pE1->pLeft, pE2->pLeft, iTab)==0
   && (pE1->op!=TK_ISNULL && pE1->op!=TK_IS)
  ){
    return 1;
  }
  return 0;
}

/*
** An instance of the following structure is used by the tree walker
** to determine if an expression can be evaluated by reference to the
** index only, without having to do a search for the corresponding
** table entry.  The IdxCover.pIdx field is the index.  IdxCover.iCur
** is the cursor for the table.
*/
struct IdxCover {
  Index *pIdx;     /* The index to be tested for coverage */
  int iCur;        /* Cursor number for the table corresponding to the index */
};

/*
** Check to see if there are references to columns in table 
** pWalker->u.pIdxCover->iCur can be satisfied using the index
** pWalker->u.pIdxCover->pIdx.
*/
static int exprIdxCover(Walker *pWalker, Expr *pExpr){
  if( pExpr->op==TK_COLUMN
   && pExpr->iTable==pWalker->u.pIdxCover->iCur
   && sqlite3ColumnOfIndex(pWalker->u.pIdxCover->pIdx, pExpr->iColumn)<0
  ){
    pWalker->eCode = 1;
    return WRC_Abort;
  }
  return WRC_Continue;
}

/*
** Determine if an index on table iCur that contains the columns in
** Bitmask m will cover the expression pExpr.  Return true if the index
** does cover the expression and false if the expression references
** table columns that are not found in the index.
**
** An index covering an expression means that the expression can be
** evaluated using only the index and without having to lookup the
** corresponding table entry.
*/
int sqlite3ExprCoveredByIndex(
  Expr *pExpr,        /* The index to be tested */
  int iCur,           /* The cursor number for the corresponding table */
  Index *pIdx         /* The index that might be used for coverage */
){
  Walker w;
  struct IdxCover xcov;
  memset(&w, 0, sizeof(w));
  xcov.iCur = iCur;
  xcov.pIdx = pIdx;
  w.xExprCallback = exprIdxCover;
  w.u.pIdxCover = &xcov;
  sqlite3WalkExpr(&w, pExpr);
  return !w.eCode;
}


/*
** An instance of the following structure is used by the tree walker
** to count references to table columns in the arguments of an 
** aggregate function, in order to implement the
** sqlite3FunctionThisSrc() routine.
*/

    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() */
    int *aiCol;                                /* array of column indexes */
    struct IdxCover *pIdxCover;                /* Check for index coverage */
  } u;
};

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

int sqlite3RunVacuum(char**, sqlite3*);
char *sqlite3NameFromToken(sqlite3*, Token*);
int sqlite3ExprCompare(Expr*, Expr*, int);
int sqlite3ExprListCompare(ExprList*, ExprList*, int);
int sqlite3ExprImpliesExpr(Expr*, Expr*, int);
void sqlite3ExprAnalyzeAggregates(NameContext*, Expr*);
void sqlite3ExprAnalyzeAggList(NameContext*,ExprList*);
int sqlite3ExprCoveredByIndex(Expr*, int iCur, Index *pIdx);
int sqlite3FunctionUsesThisSrc(Expr*, SrcList*);
Vdbe *sqlite3GetVdbe(Parse*);
#ifndef SQLITE_OMIT_BUILTIN_TEST
void sqlite3PrngSaveState(void);
void sqlite3PrngRestoreState(void);
#endif
void sqlite3RollbackAll(sqlite3*,int);

         && OptimizationEnabled(pWInfo->pParse->db, SQLITE_CoverIdxScan)
          )
      ){
        pNew->iSortIdx = b ? iSortIdx : 0;

        /* The cost of visiting the index rows is N*K, where K is
        ** between 1.1 and 3.0, depending on the relative sizes of the
        ** index and table rows. */

        pNew->rRun = rSize + 1 + (15*pProbe->szIdxRow)/pTab->szTabRow;
        if( m!=0 ){
          /* If this is a non-covering index scan, add in the cost of
          ** doing table lookups.  The cost will be 3x the number of
          ** lookups.  Take into account WHERE clause terms that can be
          ** satisfied using just the index, and that do not require a
          ** table lookup. */
          LogEst nLookup = rSize + 16;  /* Base cost:  N*3 */
          int ii;
          int iCur = pSrc->iCursor;
          WhereClause *pWC = &pWInfo->sWC;
          for(ii=0; ii<pWC->nTerm; ii++){
            WhereTerm *pTerm = &pWC->a[ii];
            if( !sqlite3ExprCoveredByIndex(pTerm->pExpr, iCur, pProbe) ){
              break;
            }
            /* pTerm can be evaluated using just the index.  So reduce
            ** the expected number of table lookups accordingly */
            if( pTerm->truthProb<=0 ){
              nLookup += pTerm->truthProb;
            }else{
              nLookup--;
              if( pTerm->eOperator & (WO_EQ|WO_IS) ) nLookup -= 19;
            }
          }
          
          pNew->rRun = sqlite3LogEstAdd(pNew->rRun, nLookup);
        }
        ApplyCostMultiplier(pNew->rRun, pTab->costMult);
        whereLoopOutputAdjust(pWC, pNew, rSize);
        rc = whereLoopInsert(pBuilder, pNew);
        pNew->nOut = rSize;
        if( rc ) break;
      }