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Re: [GENERAL] large object does not exist after pg_migrator

From: Jamie Fox <jfox(at)directcommerce(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [GENERAL] large object does not exist after pg_migrator
Date: 2009-07-14 18:59:39
Message-ID: de22a6520907141159i7dcf7e0cye28e3905d21a18a8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackers
On Mon, Jul 13, 2009 at 8:03 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> Alvaro Herrera wrote:
> > Bruce Momjian wrote:
> > > Alvaro Herrera wrote:
> > > > Bruce Momjian wrote:
> > > > > Jamie Fox wrote:
> > > >
> > > > > > I can also see that the pg_largeobject table is different, in the
> pg_restore
> > > > > > version the Rows (estimated) is 316286 and Rows (counted) is the
> same, in
> > > > > > the pg_migrator version the Rows (counted) is only 180507.
> > > >
> > > > > Wow, I didn't test large objects specifically, and I am confused
> why
> > > > > there would be a count discrepancy. I will need to do some research
> > > > > unless someone else can guess about the cause.
> > > >
> > > > Maybe pg_largeobject is not getting frozen?
> > >
> > > That would explain the change in count, but I thought we froze
> > > _everything_, and had to.
> >
> > After a quick chat with Bruce it was determined that we don't freeze
> > anything (it would be horrid for downtime if we did so in pg_migrator;
> > and it would be useless if ran anywhere else).  What we do is migrate
> > pg_clog from the old cluster to the new.  So never mind that hypothesis.
> >
> > Bruce noticed that the pg_dump/pg_migrator combo is failing to restore
> > pg_largeobject's relfrozenxid.  We're not sure how this is causing the
> > errors Jamie is seeing, because what I think should happen is that scans
> > of the table should fail with failures to open pg_clog files
> > such-and-such, but not missing tuples ...
>
> Jamie, is it possible for you to apply the attached patch to the 8.4
> server, install the new pg_dump, and run the test again to see if
> pg_largeobject is fixed?  This patch properly sets the relfrozenxid in
> the system tables for each database.
>

Sorry for the confusion, an addendum meant to be helpful fell out of this
thread during the move from -general.  I will try this patch now, but to be
clear -  the number of rows in pg_largeobject changed after I ran vacuumlo
(that eventually failed).

Here's what I have found that got broken during pg_migrate:  In two side by
side databases (an 8.3.7 copy and 8.4.0 migrated with pg_migrator) the
pg_largeobject table has the same number of rows.  However, in the 8.4
database any select for an loid in pg_largeobject returns zero rows.  If I
select all loids to a file, and compare to select all loids from 8.3.7
they're the same.  When I select != an loid it seems to exclude the one and
return the rest, but all other comparisons <, > or = return zero rows.  Or
I'm completely batty.  Dereferencing via lo_open of blob_data (an oid) in
other tables fails in the 8.4 database with 'large object xxxxid does not
exist'.

Thanks again,

Jamie

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