On 4/25/17 21:24, Michael Paquier wrote:
> Yes, and that's fine, taking a stronger lock on pg_sequence would be
> disruptive for other sessions, including the ones updating pg_sequence
> for different sequences. The point I am trying to make here is that
> the code path updating pg_sequence should make sure that the
> underlying object is properly locked first, so as the update is
> concurrent-safe because this uses simple_heap_update that assumes that
> the operation will be concurrent-safe. For example, take tablecmds.c,
> we make sure that any relation ALTER TABLE works on gets a proper lock
> with relation_open first, in what sequences would be different now
> that they have their own catalog?
Pretty much everything other than tables is a counterexample.
git grep RowExclusiveLock src/backend/commands/*.c
Only tables have an underlying object to lock. Most other DDL commands
don't have anything else to lock and run DDL under RowExclusiveLock.
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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