Re: bug report: some issues about pg_15_stable(8fa4a1ac61189efffb8b851ee77e1bc87360c445)

On Tue, 20 Feb 2024 at 14:49, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> wrote:
>
> On the face of it, the simplest fix is to tweak is_simple_union_all()
> to prevent UNION ALL subquery pullup for MERGE, forcing a
> subquery-scan plan. A quick test shows that that fixes the reported
> issue.
>
> However, that leaves the question of whether we should do the same for
> UPDATE and DELETE.
>

Attached is a patch that prevents UNION ALL subquery pullup in MERGE only.

I've re-used and extended the isolation test cases added by
1d5caec221, since it's clear that replacing the plain source relation
in those tests with a UNION ALL subquery that returns the same results
should produce the same end result. (Without this patch, the UNION ALL
subquery is pulled up, EPQ rechecking fails to re-find the match, and
a WHEN NOT MATCHED THEN INSERT action is executed instead, resulting
in a primary key violation.)

It's still not quite clear whether preventing UNION ALL subquery
pullup should also apply to UPDATE and DELETE, but I wasn't able to
find any live bug there, so I think they're best left alone.

This fixes the reported issue, though it's worth noting that
concurrent WHEN NOT MATCHED THEN INSERT actions will still lead to
duplicate rows being inserted, which is a limitation that is already
documented [1].

[1] https://www.postgresql.org/docs/current/transaction-iso.html

Regards,
Dean