Re: BUG #15900: `executor could not find named tuplestore` in triggers with transition table and row locks

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: Alex Aktsipetrov <alex(dot)akts(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org>, Andres Freund <andres(at)anarazel(dot)de>
Subject: Re: BUG #15900: `executor could not find named tuplestore` in triggers with transition table and row locks
Date: 2019-07-09 15:38:13
Message-ID: 24530.1562686693@sss.pgh.pa.us
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Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> On Tue, Jul 9, 2019 at 1:13 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>> I think it's highly likely that bug #15720 was a case of this bug and
>> would be fixed by this patch.

Agreed. I think your version of the fix is good, and you should
mention #15720 too in the commit message.

>> Alex's repro doesn't work on 11 though,
>> because EPQ is not entered at all. Which raises the question: why do
>> we need to enter EPQ after commit ad0bda5d on 12/master, for a row
>> that hasn't been updated by anyone else?

> Explanation: since ad0bda5d24ea, ExecLockRows() always calls
> EvalPlanQualBegin() which initialises the plan state, and in this case
> ExecInitNamedTuplestoreScan() errors out due to the bug. Before, you
> needed the right concurrency scenario (epq_needed) before we did that,
> as the reporter of bug #15720 discovered.

I'm quite desperately unhappy about this observation, because
EvalPlanQualBegin is a *large* amount of overhead that is usually
unnecessary, and is now going to be paid for *every locked row*
whether there's any conflict on it or not. I do not find that
acceptable. Why is it necessary to do this before finding that
there's an update conflict?

regards, tom lane

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