Date: Thu, 9 Oct 2014 12:12:30 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Adrian Klaver <adrian.klaver@aklaver.com>
Cc: jim_yates <pg@wg5jim.net>, pgsql-sql@postgresql.org
Subject: Re: could not access status of transaction pg_multixact issue
Message-ID: <20141009151229.GD7043@eldon.alvh.no-ip.org>
References: <1412791213607-5822248.post@n5.nabble.com>
 <20141008185538.GY7043@eldon.alvh.no-ip.org>
 <1412797415000-5822268.post@n5.nabble.com>
 <20141008205416.GZ7043@eldon.alvh.no-ip.org>
 <1412863620263-5822376.post@n5.nabble.com>
 <5436A34C.8090306@aklaver.com>
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Adrian Klaver wrote:
> On 10/09/2014 07:07 AM, jim_yates wrote:
> >Alvaro Herrera-9 wrote
> >>jim_yates wrote:
> >>
> >>>>A better way not involving mxid_age() would be to use pg_controldata to
> >>>>extract the current value of the mxid counter, then subtract the
> >>>current
> >>>>relminmxid from that value.
> >>>
> >>>
> >>>It's not clear which lines from pg_controldata to use for updating
> >>>pg_database.datminmxid.
> >>
> >>The one labelled NextMultiXactId.
> >>
> >>>I also assume I would do the pg_database update on a idle database.
> >>
> >>It doesn't matter, actually.  pg_database is a shared catalog, so an
> >>update would affect all the databases.
> >>
> >>--
> >>Álvaro Herrera                http://www.2ndQuadrant.com/
> >>PostgreSQL Development, 24x7 Support, Training & Services
> >
> >I tried doing the update to pg_database on my Dev server and I can't get it
> >to work.  How do I calculate the new datminmxid value?
> >
> >NextMultiXactId:  30349  relminmxid from pg_class for the table: 8376
> >
> >If I subtract the relminmxid from the nextmulixact I get 21793 which won't
> >work.
> >
> >production-copy=# update pg_database set datminmxid=21973 where
> >datname='production-copy';
> >ERROR:  column "datminmxid" is of type xid but expression is of type integer
> 
> Casting issue, try:
> 
> update pg_database set datminmxid='21973' where
> datname='production-copy';

There must have been some confusion somewhere; certainly you shouldn't
be subtracting anything.  The subtraction was just suggested as a way to
determine the age.  The value to update pg_database.datminmxid to is the
oldest one of all the relminmxid in pg_class; so if you have 8376 as the
minimum value there, that's what you set pg_database.datminmxid to.  Not
the 21973 value.

 update pg_database set datminmxid='8376' where datname='production-copy';

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


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