From ef799bdd04ee7f5d5c5fb2547633ef98c4949cfb Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 3 Jul 2020 19:01:21 -0400 Subject: [PATCH] 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 --- doc/src/sgml/fdwhandler.sgml | 8 ++++++++ src/backend/optimizer/path/allpaths.c | 3 +++ 2 files changed, 11 insertions(+) diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml index 82cc282b9a..182c04ba85 100644 --- a/doc/src/sgml/fdwhandler.sgml +++ b/doc/src/sgml/fdwhandler.sgml @@ -123,6 +123,14 @@ GetForeignRelSize(PlannerInfo *root, should be replaced if at all possible. The function may also choose to update baserel->width if it can compute a better estimate of the average result row width. + (The initial value is based on column data types and on column + average-width values measured by the last ANALYZE.) + Also, this function may update baserel->tuples if + it can compute a better estimate of the foreign table's total row count. + (The initial value is + from pg_class.reltuples + which represents the total row count seen by the + last ANALYZE.) diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index dbfa0d4f10..b601cc6a11 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -856,6 +856,9 @@ 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); } /*