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Do not compute result columns of subqueries that are never used. Make those

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columns NULL instead.  This optimization potentially resolves the enhancement
request described by [/tktview/baa5bb76c35a124c|ticket baa5bb76c35a124c].

FossilOrigin-Name: 5dec3cc0225296a043d17f73126d477d90a604f82b3180628176d8f950adbce8
This commit is contained in:
drh
2023-02-15 17:53:17 +00:00
parent bdb2ec409a
commit e3ec00ccb8
4 changed files with 78 additions and 10 deletions
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65
src/select.c
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@@ -5235,6 +5235,57 @@ static int pushDownWhereTerms(
}
#endif /* !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) */
/*
** Check to see if a subquery contains result-set columns that are
** never used. If it does, change the value of those result-set columns
** to NULL so that they do not cause unnecessary work to compute.
**
** Return the number of column that were changed to NULL.
*/
static int disableUnusedSubqueryResultColumns(SrcItem *pItem){
int nCol;
Select *pSub; /* The subquery to be simplified */
Select *pX; /* For looping over compound elements of pSub */
Table *pTab; /* The table that describes the subquery */
int j; /* Column number */
int nChng = 0; /* Number of columns converted to NULL */
assert( pItem!=0 );
assert( pItem->pTab!=0 );
pTab = pItem->pTab;
if( pTab->tabFlags & TF_Ephemeral ) return 0;
assert( pItem->pSelect!=0 );
pSub = pItem->pSelect;
assert( pSub->pEList->nExpr==pTab->nCol );
for(pX=pSub; pX; pX=pX->pPrior){
if( pX->pPrior && pX->op!=TK_ALL ){
/* This optimization does not work for compound subqueries that
** use UNION, INTERSECT, or EXCEPT. Only UNION ALL is allowed. */
return 0;
}
if( pX->pWin ){
/* This optimization does not work for subqueries that use window
** functions. */
return 0;
}
}
nCol = pTab->nCol;
for(j=0; j<nCol; j++){
Select *pX;
if( (MASKBIT(j) & pItem->colUsed)!=0 ) continue;
if( MASKBIT(j)==0 ) break;
for(pX=pSub; pX; pX=pX->pPrior) {
Expr *pY = pX->pEList->a[j].pExpr;
if( pY->op==TK_NULL ) continue;
pY->op = TK_NULL;
pX->selFlags |= SF_PushDown;
nChng++;
}
}
return nChng;
}
/*
** The pFunc is the only aggregate function in the query. Check to see
** if the query is a candidate for the min/max optimization.
@@ -7333,6 +7384,20 @@ int sqlite3Select(
TREETRACE(0x4000,pParse,p,("Push-down not possible\n"));
}
/* Convert unused result columns of the subquery into simple NULL
** expressions, to avoid unneeded searching and computation.
*/
if( disableUnusedSubqueryResultColumns(pItem) ){
#if TREETRACE_ENABLED
if( sqlite3TreeTrace & 0x4000 ){
TREETRACE(0x4000,pParse,p,
("Change unused result columns to NULL for subquery %d:\n",
pSub->selId));
sqlite3TreeViewSelect(0, p, 0);
}
#endif
}
zSavedAuthContext = pParse->zAuthContext;
pParse->zAuthContext = pItem->zName;