"Gaetano Mendola" <mendola@bigfoot.com> writes:
> select * from pg_locks where mode = 'ExclusiveLock';
> relation | database | transaction | pid | mode | granted
> ----------+----------+-------------+-------+---------------+---------
> | | 2560899 | 20404 | ExclusiveLock | t
> ExclusiveLock on what ?
Your own transaction, that's what.
If one transaction needs to wait for a specific other transaction, it
does so by attempting to grab ShareLock on the transaction ID. When
the other transaction completes (and releases its ExclusiveLock on its
ID), the waiter is released.
Offhand I think this is only used to implement waits associated with
SELECT FOR UPDATE row locking --- all other locks are on tables or
table-like objects.
regards, tom lane