On Sat, Jan 3, 2015 at 1:29 AM, Heikki Linnakangas <hlinnakan...@vmware.com> wrote: > Please explain in words of one syllable how the deadlock arises. Then, > please find a way to fix it.
I believe that the deadlock arises because there is no choke point. Exclusion constraint insertions always insert first, then check for a conflict later. Whereas with B-Trees, we check *while* inserting, at the critical point when an exclusive buffer lock is held on the first leaf page a value could be on. Two concurrent exclusion constraints inserters can easily insert at exactly the same time, and then wait on each other's xact, and then deadlock. That can't happen with B-Tree inserts because the checking and insertion happen at the same time, when that exclusive buffer lock is held. Some inserter establishes the right to insert, and then actually inserts atomically, and when it releases the buffer lock every other inserter will see for itself that it has inserted (and established the right to do so). I'm sorry, but I honestly don't see a way to fix this one. It would take a very novel approach, since exclusion constraints can work with any amgettuple AM. I briefly though about doing something crazy with the deadlock detector, but apart from anything else I think that might introduce livelock risks. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
