Re: Periodically slow inserts

Hello Tom!

Thu, 21 Oct 2010 10:55:48 -0400, you wrote:

> Gael Le Mignot <gael(at)pilotsystems(dot)net> writes:
>> The problem is when we index objects into the full-text search part of
>> the database (which a DELETE and then an INSERT into a specific table),
>> the INSERT sometimes take a long time (from 10s to 20s), but the same
>> insert (and many other similar ones) are fast (below 0.2s).

>> This slowness comes regularly, about every 200 objects indexed,
>> regardless of the frequency of the inserts.

> Hm. You didn't say which PG version you're using, but if it's >= 8.4,
> I think this may be caused by GIN's habit of queuing index insertions
> until it's accumulated a reasonable-size batch:
> http://www.postgresql.org/docs/9.0/static/gin-implementation.html#GIN-FAST-UPDATE

> While you can turn that off, I think that doing so will reduce the
> index's search efficiency over time. It might be better to schedule
> regular vacuums on the table so that the work is done by vacuum rather
> than foreground queries.

Thanks for your feedback.

It seems to be related, at least, if I increase the work_mem variable,
the slowness because bigger (up to 1 minute for a work_mem of 8mb) but
much less frequent (around 0.05% instead of 0.5% of the requests for 8mb
instead of 1mb).

So a big work_mem and a regular vacuum would do the tick, I think. Does
auto_vacuum triggers the gin index vacuuming too, or does it require a
manual vacuum ?

Regards,

--
Gaël Le Mignot - gael(at)pilotsystems(dot)net
Pilot Systems - 9, rue Desargues - 75011 Paris
Tel : +33 1 44 53 05 55 - www.pilotsystems.net
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