Horton

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

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-14T19:54:06Z
Lists: pgsql-hackers 
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                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Fix wrong data table horizon computation during backend startup.

  2. Centralize horizon determination for temp tables, fixing bug due to skew.

  3. pg_surgery: Try to stabilize regression tests.

  4. New contrib module, pg_surgery, with heap surgery functions.

  5. Set cutoff xmin more aggressively when vacuuming a temporary table.

  6. snapshot scalability: Don't compute global horizons while building snapshots.

  7. Introduce vacuum errcontext to display additional information.