Re: locks held during commit with synchronous replication

=?ISO-8859-1?Q?Torsten_F=F6rtsch?= <torsten(dot)foertsch(at)gmx(dot)net> writes:
> I noticed that with synchronous replication I often see locks like this:
> [ AccessExclusiveLock on "database 0" ]

You did not say what PG version you're talking about, but if it's recent
then this must be coming from PreCommit_Notify, which takes such a lock
while pushing entries into the shared notification-event queue:

/*
* Serialize writers by acquiring a special lock that we hold till
* after commit. This ensures that queue entries appear in commit
* order, and in particular that there are never uncommitted queue
* entries ahead of committed ones, so an uncommitted transaction
* can't block delivery of deliverable notifications.
*
* We use a heavyweight lock so that it'll automatically be released
* after either commit or abort. This also allows deadlocks to be
* detected, though really a deadlock shouldn't be possible here.
*
* The lock is on "database 0", which is pretty ugly but it doesn't
* seem worth inventing a special locktag category just for this.
* (Historical note: before PG 9.0, a similar lock on "database 0" was
* used by the flatfiles mechanism.)
*/
LockSharedObject(DatabaseRelationId, InvalidOid, 0,
AccessExclusiveLock);

This has nothing to do with synchronous replication, only with use of
LISTEN/NOTIFY.

> Does that mean that only one transaction can be committed at a time?

If they're sending notifies, yes.

regards, tom lane