Re: pg_sequence_last_value() for unlogged sequences on standbys

On Tue, Apr 30, 2024 at 09:06:04PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart(at)gmail(dot)com> writes:
>> 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
>
>> 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").
>
> Yeah, I agree with putting that logic into the function. Putting
> such conditions into the SQL of a system view is risky because it
> is really, really painful to adjust the SQL in a released version.
> You could back-patch a fix for this if done at the C level, but
> I doubt we'd go to the trouble if it's done in the view.

Good point. I'll work on a patch along these lines, then.

--
Nathan Bossart
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