Re: Odd pg dump error: cache lookup failure

I wrote:
> Wells Oliver <wells.oliver@gmail.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