Re: Incomplete freezing when truncating a relation during vacuum

How would you characterize the chances of this happening with default
*vacuum_freeze_*_age settings? Offhand, it seems you would need to encounter
this bug during each of ~10 generations of autovacuum_freeze_max_age before
the old rows actually become invisible.

On Wed, Nov 27, 2013 at 02:14:53PM +0100, Andres Freund wrote:
> With regard to fixing things up, ISTM the best bet is heap_prune_chain()
> so far. That's executed b vacuum and by opportunistic pruning and we
> know we have the appropriate locks there. Looks relatively easy to fix
> up things there. Not sure if there are any possible routes to WAL log
> this but using log_newpage()?
> I am really not sure what the best course of action is :(

Maximizing detection is valuable, and the prognosis for automated repair is
poor. I would want a way to extract tuples having xmin outside the range of
CLOG that are marked HEAP_XMIN_COMMITTED or appear on an all-visible page. At
first, I supposed we could offer a tool to blindly freeze such tuples.
However, there's no guarantee that they are in harmony with recent changes to
the database; transactions that wrongly considered those tuples invisible may
have made decisions incompatible with their existence. For example, reviving
such a tuple could violate a UNIQUE constraint if the user had already
replaced the missing row manually. A module that offers "SELECT * FROM
rows_wrongly_invisible('anytable')" would aid manual cleanup efforts.
freeze_if_wrongly_invisible(tid) would be useful, too.

Thanks,
nm

--
Noah Misch
EnterpriseDB http://www.enterprisedb.com