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Eliminate a lot of list-management overhead within join_search_one_level

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by adding a requirement that build_join_rel add new join RelOptInfos to the
appropriate list immediately at creation.  Per report from Robert Haas,
the list_concat_unique_ptr() calls that this change eliminates were taking
the lion's share of the runtime in larger join problems.  This doesn't do
anything to fix the fundamental combinatorial explosion in large join
problems, but it should push out the threshold of pain a bit further.

Note: because this changes the order in which joinrel lists are built,
it might result in changes in selected plans in cases where different
alternatives have exactly the same costs.  There is one example in the
regression tests.
This commit is contained in:
Tom Lane
2009-11-28 00:46:19 +00:00
parent fe83b975b2
commit 1a95f12702
9 changed files with 964 additions and 954 deletions
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34
src/backend/optimizer/path/allpaths.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.190 2009/11/22 14:54:31 heikki Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.191 2009/11/28 00:46:19 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -898,10 +898,15 @@ make_rel_from_joinlist(PlannerInfo *root, List *joinlist)
RelOptInfo *
standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
{
List **joinitems;
int lev;
RelOptInfo *rel;
/*
* This function cannot be invoked recursively within any one planning
* problem, so join_rel_level[] can't be in use already.
*/
Assert(root->join_rel_level == NULL);
/*
* We employ a simple "dynamic programming" algorithm: we first find all
* ways to build joins of two jointree items, then all ways to build joins
@ -909,30 +914,31 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
* joins, and so on until we have considered all ways to join all the
* items into one rel.
*
* joinitems[j] is a list of all the j-item rels. Initially we set
* joinitems[1] to represent all the single-jointree-item relations.
* root->join_rel_level[j] is a list of all the j-item rels. Initially we
* set root->join_rel_level[1] to represent all the single-jointree-item
* relations.
*/
joinitems = (List **) palloc0((levels_needed + 1) * sizeof(List *));
root->join_rel_level = (List **) palloc0((levels_needed + 1) * sizeof(List *));
joinitems[1] = initial_rels;
root->join_rel_level[1] = initial_rels;
for (lev = 2; lev <= levels_needed; lev++)
{
ListCell *x;
ListCell *lc;
/*
* Determine all possible pairs of relations to be joined at this
* level, and build paths for making each one from every available
* pair of lower-level relations.
*/
joinitems[lev] = join_search_one_level(root, lev, joinitems);
join_search_one_level(root, lev);
/*
* Do cleanup work on each just-processed rel.
*/
foreach(x, joinitems[lev])
foreach(lc, root->join_rel_level[lev])
{
rel = (RelOptInfo *) lfirst(x);
rel = (RelOptInfo *) lfirst(lc);
/* Find and save the cheapest paths for this rel */
set_cheapest(rel);
@ -946,11 +952,13 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
/*
* We should have a single rel at the final level.
*/
if (joinitems[levels_needed] == NIL)
if (root->join_rel_level[levels_needed] == NIL)
elog(ERROR, "failed to build any %d-way joins", levels_needed);
Assert(list_length(joinitems[levels_needed]) == 1);
Assert(list_length(root->join_rel_level[levels_needed]) == 1);
rel = (RelOptInfo *) linitial(joinitems[levels_needed]);
rel = (RelOptInfo *) linitial(root->join_rel_level[levels_needed]);
root->join_rel_level = NULL;
return rel;
}