ADD FOREIGN KEY locking

Hello all,

When I add a FK with a statement like this:

ALTER TABLE a ADD FOREIGN KEY (id) REFERENCES b(id);

I see a lock on table b:

select locktype,mode,granted from pg_locks, pg_stat_activity where
relation::regclass::text = 'b' AND pg_locks.pid = pg_stat_activity.pid;

locktype | relation
mode | AccessShareLock
granted | t
query | SOME LONG RUNNING QUERY WHICH SELECTS FROM b

locktype | relation
mode | AccessExclusiveLock
granted | f
query | ALTER TABLE a ADD FOREIGN KEY (id) REFERENCES b(id);

This means that my add key won't complete until my long running query does.
That seems a bit odd to me? In this database there are lots of
datawarehouse type queries running, which makes it a bit hard for me to
schedule this operation.

Is this just a manifestation of adding the key being in an ALTER TABLE,
which always needs an AccessExclusiveLock? Or am I missing some edge case
when this lock would be required in this circumstance?

No real urgency on this question, I just found it a bit strange and thought
someone might be able to shed some light.

James Sewell,
Solutions Architect
______________________________________

Level 2, 50 Queen St, Melbourne VIC 3000

*P *(+61) 3 8370 8000 *W* www.lisasoft.com *F *(+61) 3 8370 8099

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