From: Nathan Bossart <nathandbossart@gmail.com>
To: pgsql-hackers@postgresql.org
Subject: pg_sequence_last_value() for unlogged sequences on standbys
Date: Tue, 30 Apr 2024 19:57:30 -0500
Message-ID: <20240501005730.GA594666@nathanxps13> (raw)
If you create an unlogged sequence on a primary, pg_sequence_last_value()
for that sequence on a standby will error like so:
postgres=# select pg_sequence_last_value('test'::regclass);
ERROR: could not open file "base/5/16388": No such file or directory
This function is used by the pg_sequences system view, which fails with the
same error on standbys. The two options I see are:
* Return a better ERROR and adjust pg_sequences to avoid calling this
function for unlogged sequences on standbys.
* Return NULL from pg_sequence_last_value() if called for an unlogged
sequence on a standby.
As pointed out a few years ago [0], this function is undocumented, so
there's no stated contract to uphold. I lean towards just returning NULL
because that's what we'll have to put in the relevant pg_sequences field
anyway, but I can see an argument for fixing the ERROR to align with what
you see when you try to access unlogged relations on a standby (i.e.,
"cannot access temporary or unlogged relations during recovery").
Thoughts?
[0] https://postgr.es/m/CAAaqYe8JL8Et2DoO0RRjGaMvy7-C6eDH-2wHXK-gp3dOssvBkQ%40mail.gmail.com
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
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To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com
Subject: Re: pg_sequence_last_value() for unlogged sequences on standbys
In-Reply-To: <20240501005730.GA594666@nathanxps13>
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