Re: recovering from "found xmin ... from before relfrozenxid ..."

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: recovering from "found xmin ... from before relfrozenxid ..."
Date: 2020-07-14 19:54:06
Message-ID: 20200714195406.GA27279@alvherre.pgsql
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On 2020-Jul-14, Andres Freund wrote:

> Hi,
> On 2020-07-14 13:20:25 -0400, Alvaro Herrera wrote:

> > Just having the block number is already a tremendous step forward; with
> > that you can ask the customer to set a pageinspect dump of tuple
> > headers, and then the problem is obvious. Now if you want to add block
> > number to that, by all means do so.
> offset number I assume?

Eh, yeah, that.

> > One useful thing to do is to mark a tuple frozen unconditionally if it's
> > marked hinted XMIN_COMMITTED; no need to consult pg_clog in that case.
> > The attached (for 9.6) does that; IIRC it would have helped in a couple
> > of cases.
> I think it might also have hidden corruption in at least one case where
> we subsequently fixed a bug (and helped detect at least one unfixed
> bug). That should only be possible if either required clog has been
> removed, or if relfrozenxid/datfrozenxid are corrupt, right?

Yes, that's precisely the reason I never submitted it :-)

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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