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Commitef799bd

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Clamp total-tuples estimates for foreign tables to ensure planner sanity.
After running GetForeignRelSize for a foreign table, adjust rel->tuplesto be at least as large as rel->rows. This prevents bizarre behaviorin estimate_num_groups() and perhaps other places, especially in thescenario where rel->tuples is zero because pg_class.reltuples is(suggesting that ANALYZE has never been run for the table). As thingsstood, we'd end up estimating one group out of any GROUP BY on such atable, whereas the default group-count estimate is more likely to resultin a sane plan.Also, clarify in the documentation that GetForeignRelSize has the optionto override the rel->tuples value if it has a better idea of what to usethan 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 reviewDiscussion:https://postgr.es/m/CAMkU=1xNo9cnan+Npxgz0eK7394xmjmKg-QEm8wYG9P5-CcaqQ@mail.gmail.com
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‎doc/src/sgml/fdwhandler.sgml

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@@ -123,6 +123,14 @@ GetForeignRelSize(PlannerInfo *root,
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should be replaced if at all possible. The function may also choose to
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update <literal>baserel-&gt;width</literal> if it can compute a better estimate
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of the average result row width.
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(The initial value is based on column data types and on column
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average-width values measured by the last <command>ANALYZE</command>.)
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Also, this function may update <literal>baserel-&gt;tuples</literal> if
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it can compute a better estimate of the foreign table's total row count.
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(The initial value is
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from <structname>pg_class</structname>.<structfield>reltuples</structfield>
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which represents the total row count seen by the
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last <command>ANALYZE</command>.)
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</para>
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<para>

‎src/backend/optimizer/path/allpaths.c

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@@ -856,6 +856,9 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
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/* ... but do not let it set the rows estimate to zero */
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rel->rows=clamp_row_est(rel->rows);
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/* also, make sure rel->tuples is not insane relative to rel->rows */
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rel->tuples=Max(rel->tuples,rel->rows);
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}
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/*

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