Horton

Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>, "Wood, Dan" <hexpert@amazon.com>
Date: 2017-09-28T21:15:15Z
Lists: pgsql-hackers 
On Thu, Sep 28, 2017 at 1:20 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> Alvaro Herrera wrote:
>
>> Odd that it's not fixed.  I guess there's still some more work to do
>> here ...
>
> Maybe what this means is that we need to do both Dan's initially
> proposed patch (or something related to it) apart from the fixes already
> pushed.  IOW we need to put back some of the "tupkeep" business ...

We certainly do still see wrong answers to queries here:

postgres=# select ctid, xmin, xmax, * from t;
 ctid  | xmin  | xmax | id | name | x
-------+-------+------+----+------+---
 (0,1) | 21171 |    0 |  1 | 111  | 0
 (0,7) | 21177 |    0 |  3 | 333  | 5
(2 rows)

postgres=# select * from t where id = 3;
 id | name | x
----+------+---
  3 | 333  | 5
(1 row)

postgres=# set enable_seqscan = off;
SET
postgres=# select * from t where id = 3;
 id | name | x
----+------+---
(0 rows)

FWIW, I am reminded a little bit of the MultiXact/recovery bug I
reported way back in February of 2014 [1], which also had a HOT
interaction that caused index scans to give wrong answers, despite
more or less structurally sound indexes.

[1] https://www.postgresql.org/message-id/CAM3SWZTMQiCi5PV5OWHb+bYkUcnCk=O67w0cSswPvV7XfUcU5g@mail.gmail.com

-- 
Peter Geoghegan

Commits

  1. Backport addition of rs_old_rel to rewriteheap's state.

  2. Perform a lot more sanity checks when freezing tuples.

  3. Revert bogus fixes of HOT-freezing bug

  4. Fix traversal of half-frozen update chains

  5. Fix freezing of a dead HOT-updated tuple

  6. During index build, check and elog (not just Assert) for broken HOT chain.

  7. Fix WAL replay of locking an updated tuple

  8. Change the way we mark tuples as frozen.