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Avoid running build_index_pathkeys() in situations where there cannot
possibly be any useful pathkeys --- to wit, queries with neither any join clauses nor any ORDER BY request. It's nearly free to check for this case and it saves a useful fraction of the planning time for simple queries.
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3 files changed

+37
-6
lines changed

3 files changed

+37
-6
lines changed

‎src/backend/optimizer/path/indxpath.c

Lines changed: 6 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -9,7 +9,7 @@
99
*
1010
*
1111
* IDENTIFICATION
12-
* $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.219 2007/04/06 22:33:42 tgl Exp $
12+
* $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.220 2007/04/15 20:09:28 tgl Exp $
1313
*
1414
*-------------------------------------------------------------------------
1515
*/
@@ -251,6 +251,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
251251
SaOpControl saop_control)
252252
{
253253
Relids outer_relids = outer_rel ? outer_rel->relids : NULL;
254+
bool possibly_useful_pathkeys = has_useful_pathkeys(root, rel);
254255
List *result = NIL;
255256
List *all_clauses = NIL; /* not computed till needed */
256257
ListCell *ilist;
@@ -337,7 +338,8 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
337338
* relevant unless we are at top level.
338339
*/
339340
index_is_ordered = OidIsValid(index->fwdsortop[0]);
340-
if (index_is_ordered && istoplevel && outer_rel == NULL)
341+
if (index_is_ordered && possibly_useful_pathkeys &&
342+
istoplevel && outer_rel == NULL)
341343
{
342344
index_pathkeys = build_index_pathkeys(root, index,
343345
ForwardScanDirection);
@@ -369,7 +371,8 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
369371
* 4. If the index is ordered, a backwards scan might be
370372
* interesting. Again, this is only interesting at top level.
371373
*/
372-
if (index_is_ordered && istoplevel && outer_rel == NULL)
374+
if (index_is_ordered && possibly_useful_pathkeys &&
375+
istoplevel && outer_rel == NULL)
373376
{
374377
index_pathkeys = build_index_pathkeys(root, index,
375378
BackwardScanDirection);

‎src/backend/optimizer/path/pathkeys.c

Lines changed: 29 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -11,7 +11,7 @@
1111
* Portions Copyright (c) 1994, Regents of the University of California
1212
*
1313
* IDENTIFICATION
14-
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.83 2007/01/21 00:57:15 tgl Exp $
14+
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.84 2007/04/15 20:09:28 tgl Exp $
1515
*
1616
*-------------------------------------------------------------------------
1717
*/
@@ -1327,8 +1327,35 @@ truncate_useless_pathkeys(PlannerInfo *root,
13271327
* Note: not safe to modify input list destructively, but we can avoid
13281328
* copying the list if we're not actually going to change it
13291329
*/
1330-
if (nuseful == list_length(pathkeys))
1330+
if (nuseful == 0)
1331+
return NIL;
1332+
else if (nuseful == list_length(pathkeys))
13311333
return pathkeys;
13321334
else
13331335
return list_truncate(list_copy(pathkeys), nuseful);
13341336
}
1337+
1338+
/*
1339+
* has_useful_pathkeys
1340+
* Detect whether the specified rel could have any pathkeys that are
1341+
* useful according to truncate_useless_pathkeys().
1342+
*
1343+
* This is a cheap test that lets us skip building pathkeys at all in very
1344+
* simple queries. It's OK to err in the direction of returning "true" when
1345+
* there really aren't any usable pathkeys, but erring in the other direction
1346+
* is bad --- so keep this in sync with the routines above!
1347+
*
1348+
* We could make the test more complex, for example checking to see if any of
1349+
* the joinclauses are really mergejoinable, but that likely wouldn't win
1350+
* often enough to repay the extra cycles. Queries with neither a join nor
1351+
* a sort are reasonably common, though, so this much work seems worthwhile.
1352+
*/
1353+
bool
1354+
has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel)
1355+
{
1356+
if (rel->joininfo != NIL || rel->has_eclass_joins)
1357+
return true; /* might be able to use pathkeys for merging */
1358+
if (root->query_pathkeys != NIL)
1359+
return true; /* might be able to use them for ordering */
1360+
return false; /* definitely useless */
1361+
}

‎src/include/optimizer/paths.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
88
* Portions Copyright (c) 1994, Regents of the University of California
99
*
10-
* $PostgreSQL: pgsql/src/include/optimizer/paths.h,v 1.96 2007/02/16 00:14:01 tgl Exp $
10+
* $PostgreSQL: pgsql/src/include/optimizer/paths.h,v 1.97 2007/04/15 20:09:28 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -176,5 +176,6 @@ extern int pathkeys_useful_for_ordering(PlannerInfo *root, List *pathkeys);
176176
extern List *truncate_useless_pathkeys(PlannerInfo *root,
177177
RelOptInfo *rel,
178178
List *pathkeys);
179+
extern bool has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel);
179180

180181
#endif /* PATHS_H */

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