Re: question: foreign key constraints and AccessExclusive locks

On Sun, Jan 6, 2013 at 4:14 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 6 January 2013 03:08, Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> wrote:
>> When adding a foreign key constraint on tableA which references
>> tableB, why is an AccessExclusive lock on tableB necessary? Wouldn't a
>> lock that prevents writes be sufficient, or does PostgreSQL have to
>> modify *both* tables in some fashion? I'm using PostgreSQL 8.4 on
>> Linux.
>
> FKs are enforced by triggers currently. Adding triggers requires
> AccessExclusiveLock because of catalog visibility issues; you are
> right that a lower lock is eventually possible.
>
> SQLStandard requires the check to be symmetrical, so adding FKs
> requires a trigger on each table and so an AEL is placed on tableB.

I've read and re-read this a few times, and I think I understand.
However, could you clarify "you are right that a lower lock is
eventually possible" for me, please?

--
Jon