Re: Odd pg dump error: cache lookup failure

Thanks Tom, this is intriguing. I've changed our backups to do pg_dump with
verbose, and if I see this issue again I'll dig a bit with the additional
information.

On Tue, Aug 25, 2020 at 4:24 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I wrote:
> > Wells Oliver <wells(dot)oliver(at)gmail(dot)com> writes:
> >> It doesn't exist any longer, which lead me to try to think of things
> that
> >> might be dropped during the dump process.
>
> > Hm, if you're actually *dropping* matviews during the dump then it's
> > not so hard to explain this error. They'd have to be ones that were
> > selected to be dumped though.
>
> I experimented a bit to try to reproduce this problem. I cannot get
> any sort of error from REFRESH (with or without CONCURRENTLY) in
> parallel with a pg_dump. If I drop a view or matview, I can easily
> get an error, but I've not managed to reproduce one that looks like
> yours; it tends to be more like
>
> pg_dump: error: query failed: ERROR: could not open relation with OID
> 45698
>
> What I found that *would* reproduce "cache lookup failed for attribute"
> from pg_get_indexdef() is to explicitly drop a matview's index just
> before pg_dump gets to it. So I wonder if you are doing that in your
> "refresh" procedure. The timing is not terribly tight; the drop has to
> happen between where pg_dump acquires its transaction snapshot and where
> it tries to investigate the matview's indexes, which could be some while
> in a database with many objects. Also, if the transaction doing the index
> drop also takes out any exclusive locks on regular tables, that could make
> it much easier to send pg_dump down this rabbit hole, since it'd block
> on those locks till the damage was done.
>
> regards, tom lane
>

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>