On 2013-04-25 13:17:31 -0400, Tom Lane wrote: > Paul Hinze <paul.t.hi...@gmail.com> writes: > > [ multiple CREATE INDEX CONCURRENTLY commands will deadlock with each other > > ] > > Hm. I guess the reason nobody noticed this before now is that generally > the idea with CREATE INDEX CONCURRENTLY is to minimize the impact on > system load, hence you wouldn't do more than one at a time. Still, it's > surely a POLA violation that you *can't* do more than one at a time. > > The cause is that each one will wait for all older snapshots to be > gone --- and it does that before dropping its own snapshot, so that the > other ones will see it as something to be waited out too.
Makes sense. > Since we know that C.I.C. executes in its own transaction, and there > can't be more than one on the same table due to locking, it seems to me > that it'd be safe to drop our own snapshot before waiting for other > xacts to end. That is, we could just rearrange the last few steps in > DefineIndex(), taking care to save snapshot->xmin before we destroy the > snapshot so that we still have that value to pass to > GetCurrentVirtualXIDs(). > > Anybody see a flaw in that solution? Except that it still will unnecessarily wait for other CICs, just not deadlock, I don't see a problem. We could have a PROC_IN_CIC flag or something so we can ignore other index creations, but I am not sure if its worth the complication. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers