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pg_plan_advice: Document interaction with GEQO.

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The general nature of this interaction has been understood (at least
by me) since before the original commit, but I failed to explicitly
document it. I also left behind an XXX in the README which I intended
to remove before commit. Fix those things.

Reported-by: Noah Misch <noah@leadboat.com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CA+TgmoYmXy-jiP5qDhqNEiYFEBzQsArO6O2d9E8szNZqi1bePQ@mail.gmail.com
This commit is contained in:
Robert Haas
2026-09-10 13:54:30 -04:00
parent f15036f20b
commit 3117cf3882
2 changed files with 16 additions and 2 deletions
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2
contrib/pg_plan_advice/README
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@@ -263,5 +263,3 @@ good idea to incorporate that ability at some future point, as pg_hint_plan
does. However, since the primary goal of the initial development work is to be
able to induce the planner to recreate a desired plan that worked well in
the past, this has not been included in the initial development effort.
XXX Need to investigate whether and how well supplying advice works with GEQO

16
doc/src/sgml/pgplanadvice.sgml
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@@ -812,6 +812,22 @@ EXPLAIN (COSTS OFF)
force the choice of a plan which the planner refused to consider in the
first place.
</para>
<para>
One consequence of the limitation described in the previous paragraph is
that plan advice works only probabilistically when
<link linkend="geqo"><acronym>GEQO</acronym></link> is used. Even if plans
compatible with the supplied advice exist, there is no guarantee that any
of them will be a part of the randomly generated pool of candidates. If
not, the planner will generate a plan that does not comply with the
supplied advice, with some parts of that plan marked disabled. If you
supply advice that only mildly constrains join planning,
<acronym>GEQO</acronym> will usually manage to find a plan that conforms
with it; but if you wish to constrain it more substantially, you may need
to disable <acronym>GEQO</acronym> in order to obtain satisfactory results.
Note that scan advice is not affected by this limitation because it does
not constrain the join order.
</para>
</sect2>
<sect2 id="pgplanadvice-author">