Re: improve performance of pg_dump with many sequences

Nathan Bossart <nathandbossart@gmail.com> writes:
> Committed.

In the no-good-deed-goes-unpunished department: pg_dump's use
of pg_get_sequence_data() (nee pg_sequence_read_tuple()) is
evidently responsible for the complaint in bug #19365 [1]
that pg_dump can no longer survive concurrent sequence drops.

Given that that function already silently returns NULLs if the
sequence isn't readable for other reasons, I think it'd be
sane to make it silently return NULL if the sequence isn't
there anymore.  Unfortunately, that looks like it'd require
nontrivial restructuring of init_sequence().

Or maybe we could make it not use init_sequence()?  For the moment
a plain try_relation_open and check that it's a sequence should do,
but I'm not sure how that'd fit into people's plans for future
improvement of the sequence API.

There are other reasons not to like use of init_sequence in this
code path, too.  pg_dump's session will build a SeqTable entry for
every sequence in the database, which there could be a lot of,
and it will acquire RowExclusiveLock on every sequence and hold
that to the end of the dump, which seems likely to be troublesome
from a concurrency standpoint.  Since pg_get_sequence_data is a
read-only operation this lock level feels wrong.

BTW, I'm unconvinced that pg_dump behaves sanely when this function
does return nulls.  I think the ideal thing would be for it to skip
issuing setval(), but right now it looks like it will issue one with
garbage values.

            regards, tom lane

[1] https://www.postgresql.org/message-id/19365-6245240d8b926327%40postgresql.org