diff options
| author | Tom Lane <tgl@sss.pgh.pa.us> | 2010-11-18 00:30:10 -0500 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2010-11-18 00:30:10 -0500 |
| commit | 6fbc323c8042303a737028f9da7616896bccc517 (patch) | |
| tree | 8335501afc6e883ea14d0f88b3ca4f48f8a759ec /src/backend/optimizer/plan/planner.c | |
| parent | 45768d10e3abd513b4c959efeb5907798f2fac3f (diff) | |
Further fallout from the MergeAppend patch.
Fix things so that top-N sorting can be used in child Sort nodes of a
MergeAppend node, when there is a LIMIT and no intervening joins or
grouping. Actually doing this on the executor side isn't too bad,
but it's a bit messier to get the planner to cost it properly.
Per gripe from Robert Haas.
In passing, fix an oversight in the original top-N-sorting patch:
query_planner should not assume that a LIMIT can be used to make an
explicit sort cheaper when there will be grouping or aggregation in
between. Possibly this should be back-patched, but I'm not sure the
mistake is serious enough to be a real problem in practice.
Diffstat (limited to 'src/backend/optimizer/plan/planner.c')
| -rw-r--r-- | src/backend/optimizer/plan/planner.c | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c index 620888cbb86..6324bce2403 100644 --- a/src/backend/optimizer/plan/planner.c +++ b/src/backend/optimizer/plan/planner.c @@ -968,6 +968,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction) { /* No set operations, do regular planning */ List *sub_tlist; + double sub_limit_tuples; AttrNumber *groupColIdx = NULL; bool need_tlist_eval = true; QualCost tlist_cost; @@ -1120,12 +1121,27 @@ grouping_planner(PlannerInfo *root, double tuple_fraction) root->query_pathkeys = NIL; /* + * Figure out whether there's a hard limit on the number of rows that + * query_planner's result subplan needs to return. Even if we know a + * hard limit overall, it doesn't apply if the query has any + * grouping/aggregation operations. + */ + if (parse->groupClause || + parse->distinctClause || + parse->hasAggs || + parse->hasWindowFuncs || + root->hasHavingQual) + sub_limit_tuples = -1.0; + else + sub_limit_tuples = limit_tuples; + + /* * Generate the best unsorted and presorted paths for this Query (but * note there may not be any presorted path). query_planner will also * estimate the number of groups in the query, and canonicalize all * the pathkeys. */ - query_planner(root, sub_tlist, tuple_fraction, limit_tuples, + query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples, &cheapest_path, &sorted_path, &dNumGroups); /* |