SQLite

      whereLoopDelete(db, p);
    }
    sqlite3DbFree(db, pWInfo);
  }
}

/*
** Search the list of WhereLoops in *ppPrev looking for one that can be
** supplanted by pTemplate.
**
** Return NULL if the WhereLoop list contains an entry that can supplant



** pTemplate, in other words if pTemplate does not belong on the list.
**
** If pX is a WhereLoop that pTemplate can supplant, then return the



** link that points to pX.




**


** If pTemplate cannot supplant any existing element of the list but needs



** to be added to the list, then return a pointer to the tail of the list.
*/
static WhereLoop **whereLoopFindLesser(
  WhereLoop **ppPrev,












  const WhereLoop *pTemplate








){
  WhereLoop *p;


  for(p=(*ppPrev); p; ppPrev=&p->pNextLoop, p=*ppPrev){
    if( p->iTab!=pTemplate->iTab || p->iSortIdx!=pTemplate->iSortIdx ){
      /* If either the iTab or iSortIdx values for two WhereLoop are different
      ** then those WhereLoops need to be considered separately.  Neither is
      ** a candidate to replace the other. */
      continue;
    }
    /* In the current implementation, the rSetup value is either zero

       && p->nLTerm<pTemplate->nLTerm
       && (p->wsFlags & pTemplate->wsFlags & WHERE_INDEXED)!=0
       && (p->u.btree.pIndex==pTemplate->u.btree.pIndex
          || pTemplate->rRun+p->nLTerm<=p->rRun+pTemplate->nLTerm)
      ){
        /* Overwrite an existing WhereLoop with an similar one that uses
        ** more terms of the index */

        break;
      }else{
        /* There is an existing WhereLoop that is better than pTemplate */
        return 0;

      }
    }
    if( (p->prereq & pTemplate->prereq)==pTemplate->prereq
     && p->rRun>=pTemplate->rRun
     && p->nOut>=pTemplate->nOut
    ){
      /* Overwrite an existing WhereLoop with a better one: one that is
      ** better at one of (1) dependencies, (2) setup-cost, (3) run-cost
      ** or (4) number of output rows, and is no worse in any of those
      ** categories. */
      assert( p->rSetup>=pTemplate->rSetup ); /* SETUP-INVARIANT above */

      break;
    }
  }
  return ppPrev;
}

/*
** Insert or replace a WhereLoop entry using the template supplied.
**
** An existing WhereLoop entry might be overwritten if the new template
** is better and has fewer dependencies.  Or the template will be ignored
** and no insert will occur if an existing WhereLoop is faster and has
** fewer dependencies than the template.  Otherwise a new WhereLoop is
** added based on the template.
**
** If pBuilder->pOrSet is not NULL then we care about only the
** prerequisites and rRun and nOut costs of the N best loops.  That
** information is gathered in the pBuilder->pOrSet object.  This special
** processing mode is used only for OR clause processing.
**
** When accumulating multiple loops (when pBuilder->pOrSet is NULL) we
** still might overwrite similar loops with the new template if the
** template is better.  Loops may be overwritten if the following 
** conditions are met:
**
**    (1)  They have the same iTab.
**    (2)  They have the same iSortIdx.
**    (3)  The template has same or fewer dependencies than the current loop
**    (4)  The template has the same or lower cost than the current loop
**    (5)  The template uses more terms of the same index but has no additional
**         dependencies          
*/
static int whereLoopInsert(WhereLoopBuilder *pBuilder, WhereLoop *pTemplate){
  WhereLoop **ppPrev, *p;
  WhereInfo *pWInfo = pBuilder->pWInfo;
  sqlite3 *db = pWInfo->pParse->db;

  /* If pBuilder->pOrSet is defined, then only keep track of the costs
  ** and prereqs.
  */
  if( pBuilder->pOrSet!=0 ){
#if WHERETRACE_ENABLED
    u16 n = pBuilder->pOrSet->n;
    int x =
#endif
    whereOrInsert(pBuilder->pOrSet, pTemplate->prereq, pTemplate->rRun,
                                    pTemplate->nOut);
#if WHERETRACE_ENABLED /* 0x8 */
    if( sqlite3WhereTrace & 0x8 ){
      sqlite3DebugPrintf(x?"   or-%d:  ":"   or-X:  ", n);
      whereLoopPrint(pTemplate, pBuilder->pWC);
    }
#endif
    return SQLITE_OK;
  }

  /* Look for an existing WhereLoop to replace with pTemplate
  */
  ppPrev = whereLoopFindLesser(&pWInfo->pLoops, pTemplate);

  if( ppPrev==0 ){
    /* There already exists a WhereLoop on the list that is better
    ** than pTemplate, so just ignore pTemplate */
#if WHERETRACE_ENABLED /* 0x8 */
    if( sqlite3WhereTrace & 0x8 ){
      sqlite3DebugPrintf("ins-noop: ");
      whereLoopPrint(pTemplate, pBuilder->pWC);
    }
#endif
    return SQLITE_OK;  
  }else{
    p = *ppPrev;
  }

  /* If we reach this point it means that either p[] should be overwritten
  ** with pTemplate[] if p[] exists, or if p==NULL then allocate a new
  ** WhereLoop and insert it.
  */
#if WHERETRACE_ENABLED /* 0x8 */
  if( sqlite3WhereTrace & 0x8 ){
    if( p!=0 ){
      sqlite3DebugPrintf("ins-del:  ");
      whereLoopPrint(p, pBuilder->pWC);
    }
    sqlite3DebugPrintf("ins-new:  ");
    whereLoopPrint(pTemplate, pBuilder->pWC);
  }
#endif
  if( p==0 ){
    /* Allocate a new WhereLoop to add to the end of the list */
    *ppPrev = p = sqlite3DbMallocRaw(db, sizeof(WhereLoop));
    if( p==0 ) return SQLITE_NOMEM;
    whereLoopInit(p);
    p->pNextLoop = 0;
  }else{
    /* We will be overwriting WhereLoop p[].  But before we do, first
    ** go through the rest of the list and delete any other entries besides
    ** p[] that are also supplated by pTemplate */
    WhereLoop **ppTail = &p->pNextLoop;
    WhereLoop *pToDel;
    while( *ppTail ){
      ppTail = whereLoopFindLesser(ppTail, pTemplate);
      if( NEVER(ppTail==0) ) break;
      pToDel = *ppTail;
      if( pToDel==0 ) break;
      *ppTail = pToDel->pNextLoop;
#if WHERETRACE_ENABLED /* 0x8 */
      if( sqlite3WhereTrace & 0x8 ){
        sqlite3DebugPrintf("ins-del: ");
        whereLoopPrint(pToDel, pBuilder->pWC);
      }
#endif
      whereLoopDelete(db, pToDel);
    }
  }
  whereLoopXfer(db, p, pTemplate);


  if( (p->wsFlags & WHERE_VIRTUALTABLE)==0 ){
    Index *pIndex = p->u.btree.pIndex;
    if( pIndex && pIndex->tnum==0 ){
      p->u.btree.pIndex = 0;
    }
  }
  return SQLITE_OK;










}

/*
** Adjust the WhereLoop.nOut value downward to account for terms of the
** WHERE clause that reference the loop but which are not used by an
** index.
**