Jim Nasby wrote:
> On 11/10/14, 7:40 AM, Alvaro Herrera wrote:
> >Ah, right. So AFAIK we don't need to keep anything older than
> >RecentXmin or something like that -- which is not too old. If I recall
> >correctly Josh Berkus was saying in a thread about pg_multixact that it
> >used about 128kB or so in <= 9.2 for his customers; that one was also
> >limited to RecentXmin AFAIR. I think a similar volume of commit_ts data
> >would be pretty acceptable. Moreso considering that it's turned off by
> >default.
>
> FWIW, AFAICS MultiXacts are only truncated after a (auto)vacuum process is able to advance datminmxid, which will
(now)only happen when an entire relation has been scanned (which should be infrequent).
>
> I believe the low normal space usage is just an indication that most databases don't use many MultiXacts.
That's in 9.3. Prior to that, they were truncated much more often.
Maybe you've not heard enough about this commit:
commit 0ac5ad5134f2769ccbaefec73844f8504c4d6182
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Wed Jan 23 12:04:59 2013 -0300
Improve concurrency of foreign key locking
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services