Re: truncating pg_multixact/members

Robert Haas escribió:
> On Fri, Jan 3, 2014 at 9:11 AM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

> Yeah, this stuff is definitely underdocumented relative to vacuum right now.

I have added a paragraph or two. It's a (probably insufficient) start.
I would like to add a sample query to monitor usage, but I just realize
we don't have a function such as age(xid) to expose this info usefully.
We can't introduce one in 9.3 now, but probably we should do so in HEAD.

> Also, while multixactid_freeze_min_age should be low, perhaps a
> million as you suggest, multixactid_freeze_table_age should NOT be
> lowered to 3 million or anything like it. If you do that, people who
> are actually doing lots of row locking will start getting many more
> full-table scans. We want to avoid that at all cost. I'd probably
> make the default the same as for vacuum_freeze_table_age, so that
> mxids only cause extra full-table scans if they're being used more
> quickly than xids.

I agree that the freeze_table limit should not be low, but 150 million
seems too high. Not really sure what's a good value here.

Here's a first cut at this. Note I have omitted a setting equivalent to
autovacuum_freeze_max_age, but I think we should have one too.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services