From ca5e93f769e1bf5e90d080cfcc8c0368ef649a7c Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 3 Jul 2020 19:01:21 -0400 Subject: Clamp total-tuples estimates for foreign tables to ensure planner sanity. After running GetForeignRelSize for a foreign table, adjust rel->tuples to be at least as large as rel->rows. This prevents bizarre behavior in estimate_num_groups() and perhaps other places, especially in the scenario where rel->tuples is zero because pg_class.reltuples is (suggesting that ANALYZE has never been run for the table). As things stood, we'd end up estimating one group out of any GROUP BY on such a table, whereas the default group-count estimate is more likely to result in a sane plan. Also, clarify in the documentation that GetForeignRelSize has the option to override the rel->tuples value if it has a better idea of what to use than what is in pg_class.reltuples. Per report from Jeff Janes. Back-patch to all supported branches. Patch by me; thanks to Etsuro Fujita for review Discussion: https://postgr.es/m/CAMkU=1xNo9cnan+Npxgz0eK7394xmjmKg-QEm8wYG9P5-CcaqQ@mail.gmail.com --- src/backend/optimizer/path/allpaths.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'src') diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index d984da25d77..63761d55938 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -911,6 +911,9 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte) /* ... but do not let it set the rows estimate to zero */ rel->rows = clamp_row_est(rel->rows); + + /* also, make sure rel->tuples is not insane relative to rel->rows */ + rel->tuples = Max(rel->tuples, rel->rows); } /* -- cgit v1.2.3