SQLite

/*
** Advance to the next WhereTerm that matches according to the criteria
** established when the pScan object was initialized by whereScanInit().
** Return NULL if there are no more matching WhereTerms.
*/
static WhereTerm *whereScanNext(WhereScan *pScan){
  int iCur;            /* The cursor on the LHS of the term */
  int iColumn;         /* The column on the LHS of the term.  -1 for IPK */
  Expr *pX;            /* An expression being tested */
  WhereClause *pWC;    /* Shorthand for pScan->pWC */
  WhereTerm *pTerm;    /* The term being tested */
  int k = pScan->k;    /* Where to start scanning */

  while( pScan->iEquiv<=pScan->nEquiv ){
    iCur = pScan->aEquiv[pScan->iEquiv-2];
    iColumn = pScan->aEquiv[pScan->iEquiv-1];
    while( (pWC = pScan->pWC)!=0 ){
      for(pTerm=pWC->a+k; k<pWC->nTerm; k++, pTerm++){
        if( pTerm->leftCursor==iCur
         && pTerm->u.leftColumn==iColumn
         && (pScan->iEquiv<=2 || !ExprHasProperty(pTerm->pExpr, EP_FromJoin))
        ){
          if( (pTerm->eOperator & WO_EQUIV)!=0
           && pScan->nEquiv<ArraySize(pScan->aEquiv)
          ){
            int j;
            pX = sqlite3ExprSkipCollate(pTerm->pExpr->pRight);
            assert( pX->op==TK_COLUMN );
            for(j=0; j<pScan->nEquiv; j+=2){
              if( pScan->aEquiv[j]==pX->iTable
               && pScan->aEquiv[j+1]==pX->iColumn ){
                  break;
              }
            }
            if( j==pScan->nEquiv ){
              pScan->aEquiv[j] = pX->iTable;
              pScan->aEquiv[j+1] = pX->iColumn;
              pScan->nEquiv += 2;
            }
          }
          if( (pTerm->eOperator & pScan->opMask)!=0 ){
            /* Verify the affinity and collating sequence match */
            if( pScan->zCollName && (pTerm->eOperator & WO_ISNULL)==0 ){
              CollSeq *pColl;
              Parse *pParse = pWC->pWInfo->pParse;

              if( pColl==0 ) pColl = pParse->db->pDfltColl;
              if( sqlite3StrICmp(pColl->zName, pScan->zCollName) ){
                continue;
              }
            }
            if( (pTerm->eOperator & (WO_EQ|WO_IS))!=0
             && (pX = pTerm->pExpr->pRight)->op==TK_COLUMN
             && pX->iTable==pScan->aEquiv[0]
             && pX->iColumn==pScan->aEquiv[1]
            ){
              testcase( pTerm->eOperator & WO_IS );
              continue;
            }
            pScan->k = k+1;
            return pTerm;
          }
        }
      }
      pScan->pWC = pScan->pWC->pOuter;
      k = 0;
    }
    pScan->pWC = pScan->pOrigWC;
    k = 0;
    pScan->iEquiv += 2;
  }
  return 0;
}

/*
** Initialize a WHERE clause scanner object.  Return a pointer to the
** first match.  Return NULL if there are no matches.

    pScan->zCollName = pIdx->azColl[j];
  }else{
    pScan->idxaff = 0;
    pScan->zCollName = 0;
  }
  pScan->opMask = opMask;
  pScan->k = 0;
  pScan->aEquiv[0] = iCur;
  pScan->aEquiv[1] = iColumn;
  pScan->nEquiv = 2;
  pScan->iEquiv = 2;
  return whereScanNext(pScan);
}

/*
** Search for a term in the WHERE clause that is of the form "X <op> <expr>"
** where X is a reference to the iColumn of table iCur and <op> is one of
** the WO_xx operator codes specified by the op parameter.
** Return a pointer to the term.  Return 0 if not found.
**
** If pIdx!=0 then search for terms matching the iColumn-th column of pIdx
** rather than the iColumn-th column of table iCur.
**
** The term returned might by Y=<expr> if there is another constraint in
** the WHERE clause that specifies that X=Y.  Any such constraints will be
** identified by the WO_EQUIV bit in the pTerm->eOperator field.  The
** aEquiv[] array holds X and all its equivalents, with each SQL variable
** taking up two slots in aEquiv[].  The first slot is for the cursor number
** and the second is for the column number.  There are 22 slots in aEquiv[]
** so that means we can look for X plus up to 10 other equivalent values.
** Hence a search for X will return <expr> if X=A1 and A1=A2 and A2=A3
** and ... and A9=A10 and A10=<expr>.
**
** If there are multiple terms in the WHERE clause of the form "X <op> <expr>"
** then try for the one with no dependencies on <expr> - in other words where
** <expr> is a constant expression of some kind.  Only return entries of
** the form "X <op> Y" where Y is a column in another table if no terms of
** the form "X <op> <const-expr>" exist.   If no terms with a constant RHS
** exist, try to return a term that does not use WO_EQUIV.

  WhereClause *pWC;          /* WhereClause currently being scanned */
  char *zCollName;           /* Required collating sequence, if not NULL */
  char idxaff;               /* Must match this affinity, if zCollName!=NULL */
  unsigned char nEquiv;      /* Number of entries in aEquiv[] */
  unsigned char iEquiv;      /* Next unused slot in aEquiv[] */
  u32 opMask;                /* Acceptable operators */
  int k;                     /* Resume scanning at this->pWC->a[this->k] */
  int aEquiv[22];            /* Cursor,Column pairs for equivalence classes */

};

/*
** An instance of the following structure holds all information about a
** WHERE clause.  Mostly this is a container for one or more WhereTerms.
**
** Explanation of pOuter:  For a WHERE clause of the form