| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Greg Nancarrow <gregn4422(at)gmail(dot)com> |
| Cc: | Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Maxim Orlov <m(dot)orlov(at)postgrespro(dot)ru>, Michael Paquier <michael(at)paquier(dot)xyz>, Pengchengliu <pengchengliu(at)tju(dot)edu(dot)cn>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel scan with SubTransGetTopmostTransaction assert coredump |
| Date: | 2021-08-11 19:37:21 |
| Message-ID: | CA+TgmoYxFna9y-gDQjcmnN6=CXwC1CkGToZjbXZgWQFKHKGaHA@mail.gmail.com |
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On Wed, Aug 11, 2021 at 8:32 AM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> This is explained by the TransactionSnapshot being a later snapshot in
> this case.
> So this is why it seems to be wrong to call GetTransactionSnapshot()
> in InitializeParallelDSM() and use a separate, potentially later,
> snapshot than that used in the execution state for the query.
Thanks for the research. I agree with your logic here, but:
1. Then why doesn't the approach I proposed fix it?
2. Consider the case where the toplevel query is something like SELECT
complexfunc() FROM generate_series(1,10) g -- in a case like this, I
think complexfunc() can cause snapshots to be taken internally. For
example suppose we end up inside exec_eval_simple_expr, or
SPI_cursor_open_internal, in either case with read_only = false. Here
we're going to again call GetTransactionSnapshot() and then execute a
query which may use parallelism.
--
Robert Haas
EDB: http://www.enterprisedb.com
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