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

Thomas Munro <thomas.munro@gmail.com> writes:
> On Tue, Jul 9, 2019 at 1:13 PM Thomas Munro <thomas.munro@gmail.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