Re: autovacuum running for a long time on a new table with 1 row

On Fri, Jun 1, 2012 at 10:54 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Lonni J Friedman <netllama(at)gmail(dot)com> writes:
>> On Fri, Jun 1, 2012 at 10:34 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> This seems to have been noticed and fixed in HEAD:
>>> http://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=b4e0741727685443657b55932da0c06f028fbc00
>>> I wonder whether that should've been back-patched.
>
>> Thanks for your reply.  I won't even pretend to understand what that
>> fix does.  Is this behavior something that is blatantly broken, or
>> harmless, or somewhere in between?  Should I expect autovacuum to
>> eventually complete succesfully when it stumbles into this scenario?
>
> Well, the problem with the original code was that it would recheck the
> visibility map's file size anytime somebody tried to check a bit beyond
> the end of the map.  If the map isn't there (which is not an error case)
> this would result in a useless open() attempt for each table page
> scanned by vacuum.  So ordinarily I would say that yes you could expect
> autovac to complete eventually.  However ...
>
>> Before dropping & recreating the table, yes it had millions of rows,
>> and millions of updates.  But since then, all I did was insert a
>> single row, and watched autovacuum wedge itself in that seemingly
>> infinite loop.  I ended up doing a 'kill -2' on the autovacuum PID
>> that was misbehaving, disabled autovacuuming the table, and went about
>> what I needed to get done as an interim solution.
>
> ... if you really did drop and recreate the table, then at this point
> it should only have a single page, I would think.  It might be worth
> checking the actual file size.  pg_relation_size('tablename') is
> probably the quickest way.

Unfortunately, I've since inserted several million rows into this
table, so I'm guessing its too late now to check the size?