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Further tuning of the cost estimates for skip-scan loops, especially for cases

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when skip-scan loops are in competition with regular loops.

FossilOrigin-Name: a27861c28c4791e51d797aa37e9cca806cb58775
This commit is contained in:
drh
2014-10-21 18:16:21 +00:00
parent 1b131b7a7a
commit f7f2e84a9c
3 changed files with 10 additions and 14 deletions
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10
src/where.c
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@@ -3994,7 +3994,9 @@ static void whereLoopAdjustCost(const WhereLoop *p, WhereLoop *pTemplate){
if( (p->wsFlags & WHERE_INDEXED)==0 ) continue;
if( whereLoopCheaperProperSubset(p, pTemplate) ){
/* Adjust pTemplate cost downward so that it is cheaper than its
** subset p */
** 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;
@@ -4515,12 +4517,6 @@ static int whereLoopAddBtreeIndex(
pNew->aLTerm[pNew->nLTerm++] = 0;
pNew->wsFlags |= WHERE_SKIPSCAN;
nIter = pProbe->aiRowLogEst[saved_nEq] - pProbe->aiRowLogEst[saved_nEq+1];
if( pTerm ){
/* TUNING: When estimating skip-scan for a term that is also indexable,
** multiply the cost of the skip-scan by 2.0, to make it a little less
** desirable than the regular index lookup. */
nIter += 10; assert( 10==sqlite3LogEst(2) );
}
pNew->nOut -= nIter;
/* TUNING: Because uncertainties in the estimates for skip-scan queries,
** add a 1.375 fudge factor to make skip-scan slightly less likely. */