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Overview
| Comment: | Skip-ahead is now just an optimization. If it gets confused, it falls back to an incremental scan with redundancy elimination. |
|---|---|
| Downloads: | Tarball | ZIP archive |
| Timelines: | family | ancestors | descendants | both | skip-ahead-distinct |
| Files: | files | file ages | folders |
| SHA1: |
6fac0b9212ecfe5013bacd70a90fc6d4 |
| User & Date: | drh 2016-04-15 16:27:18.340 |
Context
|
2016-04-15
| ||
| 22:03 | Add the SQLITE_SkipAhead optimization setting (check-in: 87703b7661 user: drh tags: skip-ahead-distinct) | |
| 16:27 | Skip-ahead is now just an optimization. If it gets confused, it falls back to an incremental scan with redundancy elimination. (check-in: 6fac0b9212 user: drh tags: skip-ahead-distinct) | |
| 16:17 | Skip-ahead does not always work. So we still have to check for redundancy. (check-in: db5a2364eb user: drh tags: skip-ahead-distinct) | |
Changes
Changes to src/where.c.
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sqlite3ExprCacheClear(pParse);
for(i=pWInfo->nLevel-1; i>=0; i--){
int addr;
pLevel = &pWInfo->a[i];
pLoop = pLevel->pWLoop;
sqlite3VdbeResolveLabel(v, pLevel->addrCont);
if( pLevel->op!=OP_Noop ){
if( pWInfo->eDistinct==WHERE_DISTINCT_ORDERED
&& (pLoop->wsFlags & WHERE_INDEXED)!=0
){
/* 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. */
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sqlite3ExprCacheClear(pParse);
for(i=pWInfo->nLevel-1; i>=0; i--){
int addr;
pLevel = &pWInfo->a[i];
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
){
/* 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->pDistinctSet;
Index *pIdx = pLoop->u.btree.pIndex;
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;
}
}
}
}
}
if( n>0 ){
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);
VdbeCoverageIf(v, op==OP_SeekGT);
sqlite3VdbeAddOp2(v, OP_Goto, 1, pLevel->p2);
sqlite3VdbeJumpHere(v, k);
}else
#endif /* SQLITE_DISABLE_SKIPAHEAD_DISTINCT */
{
/* The common case: Advance to the next row */
sqlite3VdbeAddOp3(v, pLevel->op, pLevel->p1, pLevel->p2, pLevel->p3);
sqlite3VdbeChangeP5(v, pLevel->p5);
VdbeCoverage(v);
VdbeCoverageIf(v, pLevel->op==OP_Next);
VdbeCoverageIf(v, pLevel->op==OP_Prev);
VdbeCoverageIf(v, pLevel->op==OP_VNext);
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