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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Need Multixact Freezing Docs
Date: Wed, 16 Apr 2014 17:30:57 -0300
Message-ID: <20140416203057.GQ5822@eldon.alvh.no-ip.org> (raw)
In-Reply-To: <534EC69B.9010107@agliodbs.com>
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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


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