Re: [HACKERS] [COMMITTERS] pgsql: Fix freezing of a dead HOT-updatedtuple

Peter Geoghegan wrote:

> 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)

Yeah, oops.

> 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

Thanks for the reference.  I didn't remember this problem and it's not
(wasn't) in my list of things to look into.  Perhaps these are both the
same bug.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers