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Avoid running build_index_pathkeys() in situations where there cannot

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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.
This commit is contained in:
Tom Lane
2007-04-15 20:09:28 +00:00
parent f97d4a267a
commit fa92d21a48
3 changed files with 37 additions and 6 deletions
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31
src/backend/optimizer/path/pathkeys.c
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@ -11,7 +11,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.83 2007/01/21 00:57:15 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.84 2007/04/15 20:09:28 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -1327,8 +1327,35 @@ truncate_useless_pathkeys(PlannerInfo *root,
* Note: not safe to modify input list destructively, but we can avoid
* copying the list if we're not actually going to change it
*/
if (nuseful == list_length(pathkeys))
if (nuseful == 0)
return NIL;
else if (nuseful == list_length(pathkeys))
return pathkeys;
else
return list_truncate(list_copy(pathkeys), nuseful);
}
/*
* has_useful_pathkeys
* Detect whether the specified rel could have any pathkeys that are
* useful according to truncate_useless_pathkeys().
*
* This is a cheap test that lets us skip building pathkeys at all in very
* simple queries. It's OK to err in the direction of returning "true" when
* there really aren't any usable pathkeys, but erring in the other direction
* is bad --- so keep this in sync with the routines above!
*
* We could make the test more complex, for example checking to see if any of
* the joinclauses are really mergejoinable, but that likely wouldn't win
* often enough to repay the extra cycles. Queries with neither a join nor
* a sort are reasonably common, though, so this much work seems worthwhile.
*/
bool
has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel)
{
if (rel->joininfo != NIL || rel->has_eclass_joins)
return true; /* might be able to use pathkeys for merging */
if (root->query_pathkeys != NIL)
return true; /* might be able to use them for ordering */
return false; /* definitely useless */
}