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From: Tom Lane <[email protected]>
To: [email protected]
Cc: Richard Guo <[email protected]>
Cc: [email protected]
Cc: Robert Haas <[email protected]>
Subject: Re: BUG #19460: FULL JOIN rewriting issue on empty queries
Date: Mon, 20 Apr 2026 18:40:16 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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I wrote:
>> Pushed at cfcd57111 et al.  Thanks again for the report!

> Hmm, skink seems unhappy with this.  Looking...

Ah: equivclass.c doesn't mind letting em->em_relids be an alias
for the left_relids or right_relids of some source RestrictInfo.
That's not problematic as long as those are all constants after
construction of the EquivalenceClass, but when remove_rel_from_eclass
is trying to change things, it's a big problem.

This seems to do the trick to fix it, although I'm going to wait
for a valgrind regression run to finish before deciding this
is enough:

diff --git a/src/backend/optimizer/plan/analyzejoins.c b/src/backend/optimizer/plan/analyzejoins.c
index bfb1af614c2..03056bdf3e0 100644
--- a/src/backend/optimizer/plan/analyzejoins.c
+++ b/src/backend/optimizer/plan/analyzejoins.c
@@ -783,6 +783,8 @@ remove_rel_from_eclass(EquivalenceClass *ec, int relid, int ojrelid)
 			bms_is_member(ojrelid, cur_em->em_relids))
 		{
 			Assert(!cur_em->em_is_const);
+			/* em_relids is likely to be shared with some RestrictInfo */
+			cur_em->em_relids = bms_copy(cur_em->em_relids);
 			cur_em->em_relids = bms_del_member(cur_em->em_relids, relid);
 			cur_em->em_relids = bms_del_member(cur_em->em_relids, ojrelid);
 			if (bms_is_empty(cur_em->em_relids))


This discovery may help explain why we'd seen so few trouble
reports up to now.  At least some RestrictInfos' left/right_relids
would have indirectly gotten "fixed" by the above.

BTW, the case that is crashing the regression tests is where the above
bit reduces em_relids to empty, allowing bms_del_member to pfree it.
Now, the source RestrictInfo's left/right_relids is pointing at
garbage.  The reason this didn't cause trouble before is that if
em_relids becomes empty, we remove that EquivalenceMember altogether,
and apparently that's enough to keep us from consulting the source
RestrictInfo anymore.  But the loop over ec_sources just below does
see it, and now it is needing the left/right_relids to be valid.

			regards, tom lane

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