Re: One-off failure in "cluster" test

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: One-off failure in "cluster" test
Date: 2020-08-17 01:20:40
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Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> I wonder what caused this[1] one-off failure to see tuples in clustered order:
> ...
> I guess a synchronised scan could cause that, but I wouldn't expect one here.

Looking at its configuration, chipmunk uses

'extra_config' => {
'shared_buffers = 10MB',

which I think means that clstr_4 would be large enough to trigger a
syncscan. Ordinarily that's not a problem since no other session would
be touching clstr_4 ... but I wonder whether (a) autovacuum had decided
to look at clstr_4 and (b) syncscan can trigger on vacuum-driven scans.
(a) would explain the non-reproducibility.

I kinda think that (b), if true, is a bad idea and should be suppressed.
autovacuum would typically fail to keep up with other syncscans thanks
to vacuum delay settings, so letting it participate seems unhelpful.

regards, tom lane

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