From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | pgsql-committers(at)lists(dot)postgresql(dot)org |
Subject: | pgsql: Clamp total-tuples estimates for foreign tables to ensure planne |
Date: | 2020-07-03 23:01:40 |
Message-ID: | E1jrUge-00050D-Dr@gemulon.postgresql.org |
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Thread: | |
Lists: | pgsql-committers |
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
Branch
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REL9_5_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/d83d59e42d0ce2b2588b18d069aa13105d04447d
Modified Files
--------------
doc/src/sgml/fdwhandler.sgml | 8 ++++++++
src/backend/optimizer/path/allpaths.c | 3 +++
2 files changed, 11 insertions(+)
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