On 7.3.2015 03:26, Jeff Janes wrote:
> On Fri, Mar 6, 2015 at 5:38 PM, Tom Lane <tgl@sss.pgh.pa.us
> <mailto:tgl@sss.pgh.pa.us>> wrote:
>
> But the actual query is using a seq scan, and so it would hint the
> table in efficient sequential order, rather than hinting the table
> haphazardly in index order like probing the endpoint does.
I think this has nothing to do with the plan itself, but with the
estimation in optimizer - that looks up the range from the index in some
cases, and that may generate random I/O to the table.
>
>> Also, it's less than clear why only this particular query is
>> showing any stress. Dead rows should be a hazard for anything,
>> especially if there are enough of them to require hours to re-hint.
>> And why wouldn't autovacuum get to them first?
>
>
> Say the timing of this query is such that 10% of the parent turns
> over between invocations of this query, and that this 10% is all at
> the end of some index but random over the table heap. If autovac
> kicks in at 20% turn over, then half the time autovac does get to
> them first, and half the time it doesn't. It would be interesting to
> know if this query is bad every time it is planner, or just
> sometimes.
Yeah, this might be the reason. Another possibility is that this is part
of some large batch, and autovacuum simply did not have change to do the
work.
regards
--
Tomas Vondra http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services