Re: SERIALIZABLE with parallel query

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: SERIALIZABLE with parallel query
Date: 2017-09-25 08:57:21
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On Mon, Sep 25, 2017 at 8:37 PM, Haribabu Kommi
<kommi(dot)haribabu(at)gmail(dot)com> wrote:
> After I tune the GUC to go with sequence scan, still I am not getting the
> error
> in the session-2 for update operation like it used to generate an error for
> parallel
> sequential scan, and also it even takes some many commands until unless the
> S1
> commits.

Hmm. Then this requires more explanation because I don't expect a
difference. I did some digging and realised that the error detail
message "Reason code: Canceled on identification as a pivot, during
write." was reached in a code path that requires
SxactIsPrepared(writer) and also MySerializableXact == writer, which
means that the process believes it is committing. Clearly something
is wrong. After some more digging I realised that
ParallelWorkerMain() calls EndParallelWorkerTransaction() which calls
CommitTransaction() which calls
PreCommit_CheckForSerializationFailure(). Since the worker is
connected to the leader's SERIALIZABLEXACT, that finishes up being
marked as preparing to commit (not true!), and then the leader get
confused during that write, causing a serialization failure to be
raised sooner (though I can't explain why it should be raised then
anyway, but that's another topic). Oops. I think the fix here is
just not to do that in a worker (the worker's CommitTransaction()
doesn't really mean what it says).

Here's a version with a change that makes that conditional. This way
your test case behaves the same as non-parallel mode.

> I will continue my review on the latest patch and share any updates.


Thomas Munro

Attachment Content-Type Size
ssi-parallel-v8.patch application/octet-stream 20.1 KB

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