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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2020-07-03 19:01:22 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2020-07-03 19:01:22 -0400 |
| commit | d83d59e42d0ce2b2588b18d069aa13105d04447d (patch) | |
| tree | cd5363e862ba4e03b0a3f3c2a185e9747a2048ed | |
| parent | 57e978a622f0d9ce0e1a1d22d3e04fc2901ef5d7 (diff) | |
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
| -rw-r--r-- | doc/src/sgml/fdwhandler.sgml | 8 | ||||
| -rw-r--r-- | src/backend/optimizer/path/allpaths.c | 3 |
2 files changed, 11 insertions, 0 deletions
diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml index 6dae0bd79b3..5a868d47a36 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 <literal>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 <command>ANALYZE</command>.) + Also, this function may update <literal>baserel->tuples</literal> if + it can compute a better estimate of the foreign table's total row count. + (The initial value is + from <structname>pg_class</structname>.<structfield>reltuples</structfield> + which represents the total row count seen by the + last <command>ANALYZE</command>.) </para> <para> diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index 60fe4bfb5ec..bec549e67e4 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -587,6 +587,9 @@ 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); } /* |