Re: Query tuning - Mailing list pgsql-performance

Subbiah, Stalin wrote:
> Hello All,
>
> This query runs forever and ever. Nature of this table being lots of
> inserts/deletes/query, I vacuum it every half hour to keep the holes
> reusable and nightly once vacuum analyze to update the optimizer. We've
> got index on eventtime only. Running it for current day uses index range
> scan and it runs within acceptable time. Below is the explain of the
> query. Is the order by sequencenum desc prevents from applying limit
> optimization?
>
> explain SELECT *
> FROM EVENTLOG
> WHERE EVENTTIME>'07/23/06 16:00:00'
> AND  EVENTTIME<'08/22/06 16:00:00'
> AND  (OBJDOMAINID='tzRh39d0d91luNGT1weIUjLvFIcA'
>         OR OBJID='tzRh39d0d91luNGT1weIUjLvFIcA'
>         OR USERDOMAINID='tzRh39d0d91luNGT1weIUjLvFIcA')
> ORDER BY EVENTTIME DESC, SEQUENCENUM DESC LIMIT 500 OFFSET 0;
>
> QUERY PLAN
>
> ------------------------------------------------------------------------
> ------------------------------------------------------------------------
> ------------------------------------------------------------------------
> ------------------------------------------------------------------------
> -------------------------------------------------------------
>  Limit  (cost=15546930.29..15546931.54 rows=500 width=327)
>    ->  Sort  (cost=15546930.29..15581924.84 rows=13997819 width=327)
>          Sort Key: eventtime, sequencenum
>          ->  Seq Scan on eventlog  (cost=0.00..2332700.25 rows=13997819
> width=327)
>                Filter: ((eventtime > '2006-07-23 16:00:00'::timestamp
> without time zone) AND (eventtime < '2006-08-22 16:00:00'::timestamp
> without time zone) AND (((objdomainid)::text =
> 'tzRh39d0d91luNGT1weIUjLvFIcA'::text) OR ((objid)::text =
> 'tzRh39d0d91luNGT1weIUjLvFIcA'::text) OR ((userdomainid)::text =
> 'tzRh39d0d91luNGT1weIUjLvFIcA'::text)))
> (5 rows)
>
> Thanks,
> Stalin
> Pg version 8.0.1, suse 64bit.

Firstly you should update to 8.0.8 - because it's in the same stream you
won't need to do a dump/initdb/reload like a major version change, it
should be a simple upgrade.

Can you send explain analyze instead of just explain?

It sounds like you're not analyz'ing enough - if you're doing lots of
updates/deletes/inserts then the statistics postgresql uses to choose
whether to do an index scan or something else will quickly be outdated
and so it'll have to go back to a full table scan every time..

Can you set up autovacuum to handle that for you more regularly?

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