Horton

Re: Replication slot stats misgivings

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, vignesh C <vignesh21@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Date: 2021-04-29T03:20:00Z
Lists: pgsql-hackers 
Amit Kapila <amit.kapila16@gmail.com> writes:
> This is the first test and inserts just one small record, so how it
> can lead to spill of data. Do you mean to say that may be some
> background process has written some transaction which leads to a spill
> of data?

autovacuum, say?

> Yeah, something like this could happen. Another possibility here could
> be that before the stats collector has processed drop and create
> messages, we have enquired about the stats which lead to it giving us
> the old stats. Note, that we don't wait for 'drop' or 'create' message
> to be delivered. So, there is a possibility of the same. What do you
> think?

You should take a close look at the stats test in the main regression
tests.  We had to jump through *high* hoops to get that to be stable,
and yet it still fails semi-regularly.  This looks like pretty much the
same thing, and so I'm pessimistically inclined to guess that it will
never be entirely stable.

(At least not before the fabled stats collector rewrite, which may well
introduce some entirely new set of failure modes.)

Do we really need this test in this form?  Perhaps it could be converted
to a TAP test that's a bit more forgiving.

			regards, tom lane

Commits

  1. Doc: Update logical decoding stats information.

  2. Fix tests for replication slots stats.

  3. Update replication statistics after every stream/spill.

  4. Fix the computation of slot stats for 'total_bytes'.

  5. Another try to fix the test case added by commit f5fc2f5b23.

  6. Use HTAB for replication slot statistics.

  7. Fix test case added by commit f5fc2f5b23.

  8. Add information of total data processed to replication slot stats.

  9. Use NameData datatype for slotname in stats.