Re: pg_sequence_last_value() for unlogged sequences on standbys

Nathan Bossart <nathandbossart@gmail.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.

            regards, tom lane