SQLite

** This module contains C code that generates VDBE code used to process
** the WHERE clause of SQL statements.  This module is reponsible for
** generating the code that loops through a table looking for applicable
** rows.  Indices are selected and used to speed the search when doing
** so is applicable.  Because this module is responsible for selecting
** indices, you might also think of this module as the "query optimizer".
**
** $Id: where.c,v 1.158 2005/07/29 19:43:58 drh Exp $
*/
#include "sqliteInt.h"

/*
** The number of bits in a Bitmask.  "BMS" means "BitMask Size".
*/
#define BMS  (sizeof(Bitmask)*8)

** bits in the Bitmask.  So, in the example above, the cursor numbers
** would be mapped into integers 0 through 7.
*/
typedef struct WhereTerm WhereTerm;
struct WhereTerm {
  Expr *pExpr;            /* Pointer to the subexpression */
  u16 idx;                /* Index of this term in pWC->a[] */
  i16 iPartner;           /* Disable pWC->a[iPartner] when this term disabled */
  i16 leftCursor;         /* Cursor number of X in "X <op> <expr>" */
  i16 leftColumn;         /* Column number of X in "X <op> <expr>" */
  u16 operator;           /* A WO_xx value describing <op> */
  u8 flags;               /* Bit flags.  See below */
  u8 nPartner;            /* Number of partners that must disable us */
  WhereClause *pWC;       /* The clause this term is part of */
  Bitmask prereqRight;    /* Bitmask of tables used by pRight */
  Bitmask prereqAll;      /* Bitmask of tables referenced by p */
};

/*
** Allowed values of WhereTerm.flags
*/
#define TERM_DYNAMIC    0x01   /* Need to call sqlite3ExprDelete(pExpr) */
#define TERM_VIRTUAL    0x02   /* Added by the optimizer.  Do not code */
#define TERM_CODED      0x04   /* This term is already coded */
#define TERM_PARTNERED  0x08   /* Has a virtual partner */
#define TERM_OR_OK      0x10   /* Used during OR-clause processing */

/*
** An instance of the following structure holds all information about a
** WHERE clause.  Mostly this is a container for one or more WhereTerms.
*/
struct WhereClause {

  }
  pTerm = &pWC->a[pWC->nTerm];
  pTerm->idx = pWC->nTerm;
  pWC->nTerm++;
  pTerm->pExpr = p;
  pTerm->flags = flags;
  pTerm->pWC = pWC;
  pTerm->iPartner = -1;
  return pTerm;
}

/*
** This routine identifies subexpressions in the WHERE clause where
** each subexpression is separate by the AND operator or some other
** operator specified in the op parameter.  The WhereClause structure

  Bitmask prereqAll;
  int idxRight;

  prereqLeft = exprTableUsage(pMaskSet, pExpr->pLeft);
  pTerm->prereqRight = exprTableUsage(pMaskSet, pExpr->pRight);
  pTerm->prereqAll = prereqAll = exprTableUsage(pMaskSet, pExpr);
  pTerm->leftCursor = -1;
  pTerm->iPartner = -1;
  pTerm->operator = 0;
  idxRight = -1;
  if( allowedOp(pExpr->op) && (pTerm->prereqRight & prereqLeft)==0 ){
    Expr *pLeft = pExpr->pLeft;
    Expr *pRight = pExpr->pRight;
    if( pLeft->op==TK_COLUMN ){
      pTerm->leftCursor = pLeft->iTable;
      pTerm->leftColumn = pLeft->iColumn;
      pTerm->operator = operatorMask(pExpr->op);
    }
    if( pRight && pRight->op==TK_COLUMN ){
      WhereTerm *pNew;
      Expr *pDup;
      if( pTerm->leftCursor>=0 ){
        pDup = sqlite3ExprDup(pExpr);
        pNew = whereClauseInsert(pTerm->pWC, pDup, TERM_VIRTUAL|TERM_DYNAMIC);
        if( pNew==0 ) return;
        pNew->iPartner = pTerm->idx;
        pTerm->nPartner = 1;
        pTerm->flags |= TERM_PARTNERED;
      }else{
        pDup = pExpr;
        pNew = pTerm;
      }
      exprCommute(pDup);
      pLeft = pDup->pLeft;
      pNew->leftCursor = pLeft->iTable;

      Expr *pNewExpr;
      WhereTerm *pNewTerm;
      pNewExpr = sqlite3Expr(ops[i], sqlite3ExprDup(pExpr->pLeft),
                             sqlite3ExprDup(pList->a[i].pExpr), 0);
      pNewTerm = whereClauseInsert(pTerm->pWC, pNewExpr,
                                   TERM_VIRTUAL|TERM_DYNAMIC);
      exprAnalyze(pSrc, pMaskSet, pNewTerm);
      pNewTerm->iPartner = pTerm->idx;
    }
    pTerm->nPartner = 2;
  }

  /* Attempt to convert OR-connected terms into an IN operator so that
  ** they can make use of indices.
  */
  else if( pExpr->op==TK_OR ){
    int ok;

      ok = iCursor>=0;
      for(i=sOr.nTerm-1, pOrTerm=sOr.a; i>=0 && ok; i--, pOrTerm++){
        if( pOrTerm->operator!=WO_EQ ){
          goto or_not_possible;
        }
        if( pOrTerm->leftCursor==iCursor && pOrTerm->leftColumn==iColumn ){
          pOrTerm->flags |= TERM_OR_OK;
        }else if( (pOrTerm->flags & TERM_PARTNERED)!=0 ||
                    ((pOrTerm->flags & TERM_VIRTUAL)!=0 &&
                     (sOr.a[pOrTerm->iPartner].flags & TERM_OR_OK)!=0) ){
          pOrTerm->flags &= ~TERM_OR_OK;
        }else{
          ok = 0;
        }
      }
    }while( !ok && (sOr.a[j++].flags & TERM_PARTNERED)!=0 && j<sOr.nTerm );
    if( ok ){
      ExprList *pList = 0;
      Expr *pNew, *pDup;
      for(i=sOr.nTerm-1, pOrTerm=sOr.a; i>=0 && ok; i--, pOrTerm++){
        if( (pOrTerm->flags & TERM_OR_OK)==0 ) continue;
        pDup = sqlite3ExprDup(pOrTerm->pExpr->pRight);
        pList = sqlite3ExprListAppend(pList, pDup, 0);

*/
static void disableTerm(WhereLevel *pLevel, WhereTerm *pTerm){
  if( pTerm
      && (pTerm->flags & TERM_CODED)==0
      && (pLevel->iLeftJoin==0 || ExprHasProperty(pTerm->pExpr, EP_FromJoin))
  ){
    pTerm->flags |= TERM_CODED;
    if( pTerm->iPartner>=0 ){
      WhereTerm *pOther = &pTerm->pWC->a[pTerm->iPartner];
      if( (--pOther->nPartner)<=0 ){
        disableTerm(pLevel, pOther);
      }
    }
  }
}

/*