Date: Wed, 16 Apr 2014 17:30:57 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Need Multixact Freezing Docs
Message-ID: <20140416203057.GQ5822@eldon.alvh.no-ip.org>
References: <534DA3D1.2010000@agliodbs.com>
 <WM!e71c36af513bace4ff046f7349231f370d2b74c4adee803b2eb3ed330aada82b5badd712b47f9556d3fa55d2ab73a3e0!@asav-3.01.com>
 <534DB22A.2040909@agliodbs.com>
 <20140416133233.GL5822@eldon.alvh.no-ip.org>
 <WM!a70b2681d2ac0ed55eb2d3eed44e6730e9c71b5fad8c5320503fde8b18a03fd98a6af6244821ca39d47c1d9e758c45ff!@asav-2.01.com>
 <534EC69B.9010107@agliodbs.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <534EC69B.9010107@agliodbs.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Josh Berkus wrote:
> 
> > You can see the current multixact value in pg_controldata output.  Keep
> > timestamped values of that somewhere (a table?) so that you can measure
> > consumption rate.  I don't think we provide SQL-level access to those
> > values.
> 
> Bleh.  Do we provide SQL-level access in 9.4?  If not, I think that's a
> requirement before release.

Yeah, good idea.  Want to propose a patch?

> >> Also: how do I check the multixact age of a table?  There doesn't seem
> >> to be any data for this ...
> > 
> > pg_class.relminmxid is the oldest multixact value that might be present
> > in a table.
> 
> On every database I've tested, age(relminmxid) returns int_max.  So this
> is apparently broken.

Hmm, are you sure it's INT_MAX and not 4244967297?  Heikki reported
that: http://www.postgresql.org/message-id/52401AEA.9000608@vmware.com
The absolute value is not important; I think that's mostly harmless.  I
don't think applying age() to a multixact value is meaningful, though;
that's only good for Xids.

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


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers