From: Tom Lane <tgl@sss.pgh.pa.us>
To: Eric Raskin <eraskin@paslists.com>
cc: pgsql-performance@lists.postgresql.org
Subject: Re: Adding nextval() to a select caused hang/very slow execution
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Comments: In-reply-to Eric Raskin <eraskin@paslists.com>
	message dated "Wed, 04 Nov 2020 12:10:21 -0500"
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Date: Wed, 04 Nov 2020 13:04:18 -0500
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Eric Raskin <eraskin@paslists.com> writes:
> I have a strange situation where a base query completes in about 30 seconds
> but if I add a nextval() call to the select it never completes.  There are
> other processes running that are accessing the same sequence, but I thought
> that concurrency was not an issue for sequences (other than skipped
> values).

Shouldn't be, probably ... but did you check to see if the query is
blocked on a lock?  (See pg_stat_activity or pg_locks views.)

> The only change that
> causes it to be extremely slow or hang (can't tell which) is that I changed
> the select from:
> select unnest(....
> to
> select nextval('sbowner.idgen'), unnest(....

Without seeing the complete query it's hard to say much.  But if
this isn't the topmost select list, maybe what's happening is that
the presence of a volatile function in a sub-select is defeating
some key plan optimization.  Did you compare plain EXPLAIN (w/out
ANALYZE) output for the two cases, to see if the plan shape changes?

			regards, tom lane