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Comment:Enhance whereLoopCheaperProperSubset(X,Y) so that it does not report true if X uses skip-scan less than Y, since in that case X might deserve to be cheaper even if it is a proper subset.
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SHA1: c106b755369c1f8546e897ecd2ac56fd09d6e885
User & Date: drh 2014-11-05 13:13:13
Context
2014-11-05
14:19
Add a test case to check that the previous commit is effective. check-in: 948d6e5d user: dan tags: trunk
13:13
Enhance whereLoopCheaperProperSubset(X,Y) so that it does not report true if X uses skip-scan less than Y, since in that case X might deserve to be cheaper even if it is a proper subset. check-in: c106b755 user: drh tags: trunk
09:07
Add the ".scanstats on" command to the shell tool. Executing this command causes the shell tool to print values from sqlite3_stmt_scanstatus() after each query is run. check-in: 7974c0ed user: dan tags: trunk
Changes
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Changes to src/where.c.

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

/*
** Return TRUE if both of the following are true:
**
**   (1)  X has the same or lower cost that Y
**   (2)  X is a proper subset of Y

**
** By "proper subset" we mean that X uses fewer WHERE clause terms
** than Y and that every WHERE clause term used by X is also used
** by Y.
**
** If X is a proper subset of Y then Y is a better choice and ought
** to have a lower cost.  This routine returns TRUE when that cost 
** relationship is inverted and needs to be adjusted.


*/
static int whereLoopCheaperProperSubset(
  const WhereLoop *pX,       /* First WhereLoop to compare */
  const WhereLoop *pY        /* Compare against this WhereLoop */
){
  int i, j;
  if( pX->nLTerm-pX->nSkip >= pY->nLTerm-pY->nSkip ){
    return 0; /* X is not a subset of Y */
  }

  if( pX->rRun >= pY->rRun ){
    if( pX->rRun > pY->rRun ) return 0;    /* X costs more than Y */
    if( pX->nOut > pY->nOut ) return 0;    /* X costs more than Y */
  }
  for(i=pX->nLTerm-1; i>=0; i--){
    if( pX->aLTerm[i]==0 ) continue;
    for(j=pY->nLTerm-1; j>=0; j--){
................................................................................
static void whereLoopAdjustCost(const WhereLoop *p, WhereLoop *pTemplate){
  if( (pTemplate->wsFlags & WHERE_INDEXED)==0 ) return;
  for(; p; p=p->pNextLoop){
    if( p->iTab!=pTemplate->iTab ) continue;
    if( (p->wsFlags & WHERE_INDEXED)==0 ) continue;
    if( whereLoopCheaperProperSubset(p, pTemplate) ){
      /* Adjust pTemplate cost downward so that it is cheaper than its 
      ** subset p.  Except, do not adjust the cost estimate downward for
      ** a loop that skips more columns. */
      if( pTemplate->nSkip>p->nSkip ) continue;
      WHERETRACE(0x80,("subset cost adjustment %d,%d to %d,%d\n",
                       pTemplate->rRun, pTemplate->nOut, p->rRun, p->nOut-1));
      pTemplate->rRun = p->rRun;
      pTemplate->nOut = p->nOut - 1;
    }else if( whereLoopCheaperProperSubset(pTemplate, p) ){
      /* Adjust pTemplate cost upward so that it is costlier than p since
      ** pTemplate is a proper subset of p */







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

/*
** Return TRUE if all of the following are true:
**
**   (1)  X has the same or lower cost that Y
**   (2)  X is a proper subset of Y
**   (3)  X skips at least as many columns as Y
**
** By "proper subset" we mean that X uses fewer WHERE clause terms
** than Y and that every WHERE clause term used by X is also used
** by Y.
**
** If X is a proper subset of Y then Y is a better choice and ought
** to have a lower cost.  This routine returns TRUE when that cost 
** relationship is inverted and needs to be adjusted.  The third rule
** was added because if X uses skip-scan less than Y it still might
** deserve a lower cost even if it is a proper subset of Y.
*/
static int whereLoopCheaperProperSubset(
  const WhereLoop *pX,       /* First WhereLoop to compare */
  const WhereLoop *pY        /* Compare against this WhereLoop */
){
  int i, j;
  if( pX->nLTerm-pX->nSkip >= pY->nLTerm-pY->nSkip ){
    return 0; /* X is not a subset of Y */
  }
  if( pY->nSkip > pX->nSkip ) return 0;
  if( pX->rRun >= pY->rRun ){
    if( pX->rRun > pY->rRun ) return 0;    /* X costs more than Y */
    if( pX->nOut > pY->nOut ) return 0;    /* X costs more than Y */
  }
  for(i=pX->nLTerm-1; i>=0; i--){
    if( pX->aLTerm[i]==0 ) continue;
    for(j=pY->nLTerm-1; j>=0; j--){
................................................................................
static void whereLoopAdjustCost(const WhereLoop *p, WhereLoop *pTemplate){
  if( (pTemplate->wsFlags & WHERE_INDEXED)==0 ) return;
  for(; p; p=p->pNextLoop){
    if( p->iTab!=pTemplate->iTab ) continue;
    if( (p->wsFlags & WHERE_INDEXED)==0 ) continue;
    if( whereLoopCheaperProperSubset(p, pTemplate) ){
      /* Adjust pTemplate cost downward so that it is cheaper than its 
      ** subset p. */


      WHERETRACE(0x80,("subset cost adjustment %d,%d to %d,%d\n",
                       pTemplate->rRun, pTemplate->nOut, p->rRun, p->nOut-1));
      pTemplate->rRun = p->rRun;
      pTemplate->nOut = p->nOut - 1;
    }else if( whereLoopCheaperProperSubset(pTemplate, p) ){
      /* Adjust pTemplate cost upward so that it is costlier than p since
      ** pTemplate is a proper subset of p */