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Overview
| Comment: | Only use the skip-ahead-distinct optimization if the index has been analyzed and we know that a skip-head is likely to skip over at least 11 rows. The magic number 11 was determined by experimentation. |
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| Downloads: | Tarball | ZIP archive |
| Timelines: | family | ancestors | descendants | both | skip-ahead-distinct |
| Files: | files | file ages | folders |
| SHA3-256: |
0cf16decd534bf2d66620c293f8c8987 |
| User & Date: | drh 2017-04-13 13:01:59.793 |
Context
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2017-04-13
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| 18:33 | New test cases and minor fixes for the optimization on this branch. (check-in: 70ac9ea1a6 user: dan tags: skip-ahead-distinct) | |
| 13:01 | Only use the skip-ahead-distinct optimization if the index has been analyzed and we know that a skip-head is likely to skip over at least 11 rows. The magic number 11 was determined by experimentation. (check-in: 0cf16decd5 user: drh tags: skip-ahead-distinct) | |
| 01:19 | Forward port the skip-ahead-distinct branch which was abandoned for some reason that I do not recall. This port should have been achived by a merge of trunk into the previous head of skip-ahead-distinct, but that did not work. So I had to manually "rebase" the changes. (check-in: 132339a1fb user: drh tags: skip-ahead-distinct) | |
Changes
Changes to src/where.c.
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pLoop = pLevel->pWLoop;
sqlite3VdbeResolveLabel(v, pLevel->addrCont);
if( pLevel->op!=OP_Noop ){
#ifndef SQLITE_DISABLE_SKIPAHEAD_DISTINCT
int n = -1;
int j, k, op;
int r1 = pParse->nMem+1;
if( pWInfo->eDistinct==WHERE_DISTINCT_ORDERED
&& (pLoop->wsFlags & WHERE_INDEXED)!=0
&& OptimizationEnabled(db, SQLITE_SkipAhead)
){
/* This is the Skip-ahead optimization. When doing a DISTINCT query
** that has WHERE_DISTINCT_ORDERED, use OP_SkipGT/OP_SkipLT to skip
** over all duplicate entries, rather than visiting all duplicates
** using OP_Next/OP_Prev. */
ExprList *pX = pWInfo->pResultSet;
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pLoop = pLevel->pWLoop;
sqlite3VdbeResolveLabel(v, pLevel->addrCont);
if( pLevel->op!=OP_Noop ){
#ifndef SQLITE_DISABLE_SKIPAHEAD_DISTINCT
int n = -1;
int j, k, op;
int r1 = pParse->nMem+1;
Index *pIdx;
if( pWInfo->eDistinct==WHERE_DISTINCT_ORDERED
&& (pLoop->wsFlags & WHERE_INDEXED)!=0
&& OptimizationEnabled(db, SQLITE_SkipAhead)
&& (pIdx = pLoop->u.btree.pIndex)->hasStat1
){
/* This is the Skip-ahead optimization. When doing a DISTINCT query
** that has WHERE_DISTINCT_ORDERED, use OP_SkipGT/OP_SkipLT to skip
** over all duplicate entries, rather than visiting all duplicates
** using OP_Next/OP_Prev. */
ExprList *pX = pWInfo->pResultSet;
for(j=0; j<pX->nExpr; j++){
Expr *pE = sqlite3ExprSkipCollate(pX->a[j].pExpr);
if( pE->op==TK_COLUMN ){
if( pE->iTable!=pLevel->iTabCur ) continue;
k = 1+sqlite3ColumnOfIndex(pIdx, pE->iColumn);
if( k>n ) n = k;
}else if( pIdx->aColExpr ){
for(k=n+1; k<pIdx->nKeyCol; k++){
Expr *pI = pIdx->aColExpr->a[k].pExpr;
if( pI && sqlite3ExprCompare(pE,pI,0)<2 ){
n = k+1;
break;
}
}
}
}
}
/* TUNING: Only try to skip ahead using OP_Seek if we expect to
** skip over 11 or more rows. Otherwise, OP_Next is just as fast.
*/
assert( 36==sqlite3LogEst(12) );
if( n>0 && pIdx->aiRowLogEst[n]>=36 ){
for(j=0; j<n; j++){
sqlite3VdbeAddOp3(v, OP_Column, pLevel->iIdxCur, j, r1+j);
}
pParse->nMem += n;
op = pLevel->op==OP_Prev ? OP_SeekLT : OP_SeekGT;
k = sqlite3VdbeAddOp4Int(v, op, pLevel->iIdxCur, 0, r1, n);
VdbeCoverageIf(v, op==OP_SeekLT);
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