Horton

Re: Old row version in hot chain become visible after a freeze

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: "Wong, Yi Wen" <yiwong@amazon.com>
Cc: Michael Paquier <michael.paquier@gmail.com>, Peter Geoghegan <pg@bowt.ie>, "Wood, Dan" <hexpert@amazon.com>, "pgsql-bugs@postgresql.org" <pgsql-bugs@postgresql.org>
Date: 2017-09-08T16:42:44Z
Lists: pgsql-bugs 
Wong, Yi Wen wrote:

> There is actually another case where HEAPTUPLE_DEAD tuples may be kept and have
> prepare_freeze skipped on them entirely.
> 
> lazy_record_dead_tuple may fail to record the heap for later pruning
> for lazy_vacuum_heap if there is already a sufficiently large number of dead tuples
> in the array:

Hmm, ouch, good catch.

AFAICS this is a shouldn't-happen condition, since we bail out of the
loop pessimistically as soon as we would be over the array limit if the
next page were to be full of dead tuples (i.e., we never give the chance
for overflow to actually happen).  So unless I misunderstand, this could
only fail if you give maint_work_mem smaller than necessary for one
pageful of dead tuples, which should be about 1800 bytes ...

If we wanted to be very sure about this we could add a test and perhaps
abort the vacuum, but I'm not sure it's worth the trouble.


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

Commits

  1. Fix freezing of a dead HOT-updated tuple

  2. Fix potential data corruption during freeze

  3. Clarify the contract of partition_rbound_cmp().