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Do not flatten subqueries that contain an ORDER BY or GROUP BY clause and

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can be implemented using a co-routine.

FossilOrigin-Name: 042d655dd9002e8b89a798ad955b0285891aecf79f6978c5312e70ffe0609a46
This commit is contained in:
drh
2017-09-28 20:06:53 +00:00
parent 2e9d706252
commit 648fe49f96
3 changed files with 29 additions and 7 deletions
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19
src/select.c
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@@ -5205,6 +5205,25 @@ int sqlite3Select(
goto select_end;
}
/* If the subquery contains an ORDER BY or GROUP BY clause and if
** it will be implemented as a co-routine, then do not flatten. This
** restriction allows SQL constructs like this:
**
** SELECT expensive_function(x)
** FROM (SELECT x FROM tab ORDER BY y LIMIT 10);
**
** The expensive_function() is only computed on the 10 rows that
** are output, rather than every row of the table.
*/
if( (pSub->pOrderBy!=0 || pSub->pGroupBy!=0)
&& i==0
&& (pTabList->nSrc==1
|| (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)
&& OptimizationEnabled(db, SQLITE_SubqCoroutine)
){
continue;
}
isAggSub = (pSub->selFlags & SF_Aggregate)!=0;
if( flattenSubquery(pParse, p, i, isAgg, isAggSub) ){
/* This subquery can be absorbed into its parent. */