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

From: Andres Freund <andres(at)anarazel(dot)de>
To: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
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 19:36:55
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On 2020-07-14 13:20:25 -0400, Alvaro Herrera wrote:
> On 2020-Jul-13, Andres Freund wrote:
> > Hi,
> >
> > On 2020-07-13 17:12:18 -0400, Robert Haas wrote:
> > > 1. There's nothing to identify the tuple that has the problem, and no
> > > way to know how many more of them there might be. Back-patching
> > > b61d161c146328ae6ba9ed937862d66e5c8b035a would help with the first
> > > part of this.
> >
> > Not fully, I'm afraid. Afaict it doesn't currently tell you the item
> > pointer offset, just the block numer, right? We probably should extend
> > it to also include the offset...
> 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?

> 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?


Andres Freund

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