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Attempt to omit ORDER BY clauses from FROM-clause subqueries if those ORDER BY

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clauses do not affect the output.  See
[forum:/forumpost/2d76f2bcf65d256a|forum thread 2d76f2bcf65d256a] for
discussion.  This can help the query flattener in
some cases, resulting in faster query plans.  The current implemention does
not always work.

FossilOrigin-Name: ef97c3e7c3ea2cf1a4db6591328fe7ce3f1d189afc2d578159135824ec89e620
This commit is contained in:
drh
2021-07-15 19:29:43 +00:00
parent 480f5e3e6e
commit bb30123178
7 changed files with 98 additions and 57 deletions
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25
src/select.c
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@@ -6384,6 +6384,31 @@ int sqlite3Select(
if( (pSub->selFlags & SF_Aggregate)!=0 ) continue;
assert( pSub->pGroupBy==0 );
/* If a FROM-clause subquery has an ORDER BY clause that is not
** really doing anything, then delete it now so that it does not
** interfere with query flattening.
**
** Beware of these cases where the ORDER BY clause may not be safely
** omitted:
**
** (1) There is also a LIMIT clause
** (2) The subquery was added to help with window-function
** processing
** (3) The outer query uses an aggregate function other than
** the built-in count(), min(), or max().
*/
if( pSub->pOrderBy!=0
&& pSub->pLimit==0 /* Condition (1) */
&& (pSub->selFlags & SF_OrderByReqd)==0 /* Condition (2) */
&& (p->selFlags & SF_OrderByReqd)==0 /* Condition (3) */
&& OptimizationEnabled(db, SQLITE_OmitOrderBy)
){
SELECTTRACE(0x100,pParse,p,
("omit superfluous ORDER BY on %r FROM-clause subquery\n",i+1));
sqlite3ExprListDelete(db, pSub->pOrderBy);
pSub->pOrderBy = 0;
}
/* If the outer query contains a "complex" result set (that is,
** if the result set of the outer query uses functions or subqueries)
** and if the subquery contains an ORDER BY clause and if