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: 2020-07-14T17:20:25Z
Lists: pgsql-hackers

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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.

FWIW I do support the idea of backpatching the vacuum errcontext commit.

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.

-- 
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.