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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2020-07-03 19:01:21 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2020-07-03 19:01:21 -0400 |
| commit | 153c14cdd714381b186d3e3a2272d29f4717be9e (patch) | |
| tree | 1d6f1169690c8b42b5b5e7ffc71466f910e0fbac | |
| parent | 8b83e01607a85c1099dc08f0f95ce22f715bfa26 (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 6587678af2b..1e997c218b9 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</literal> 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 58269374056..8e8f43735e0 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); } /* |