Re: LIKE Query performance

Only wildspeed http://www.sai.msu.su/~megera/wiki/wildspeed
has index support for %text%
But, it has limitations.

Oleg
On Tue, 27 Jan 2009, Hari, Balaji wrote:

> Hi,
>
> I am relatively new to PostgreSQL(8.1) and facing the following problem.
>
> We have indexes defined on timestamp and description (create index description_idx on event using btree (description varchar_pattern_ops))
>
> EXPLAIN ANALYZE SELECT event_id, category, current_session_number, description, event_type_id, realm_name, root_session_number, severity, source_name, target_key, target_name, timestamp, jdo_version FROM event WHERE description like '%mismatch%' ORDER BY timestamp desc;
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------
> Sort (cost=36267.09..36272.73 rows=2256 width=314) (actual time=19255.075..20345.774 rows=647537 loops=1)
> Sort Key: "timestamp"
> Sort Method: external merge Disk: 194080kB
> -> Seq Scan on event (cost=0.00..36141.44 rows=2256 width=314) (actual time=0.080..1475.041 rows=647537 loops=1)
> Filter: ((description)::text ~~ '%mismatch%'::text)
> Total runtime: 22547.292 ms
> (6 rows)
>
> But startsWith query use indexes.
>
> EXPLAIN ANALYZE SELECT event_id, category, current_session_number, description, event_type_id, realm_name, root_session_number, severity, source_name, target_key, target_name, timestamp, jdo_version FROM event WHERE description like 'mismatch%' ORDER BY timestamp desc;
> QUERY PLAN
> -------------------------------------------------------------------------------------------------------------------------------
> Sort (cost=9.26..9.27 rows=1 width=314) (actual time=0.766..0.766 rows=0 loops=1)
> Sort Key: "timestamp"
> Sort Method: quicksort Memory: 17kB
> -> Index Scan using description_idx on event (cost=0.00..9.25 rows=1 width=314) (actual time=0.741..0.741 rows=0 loops=1)
> Index Cond: (((description)::text ~>=~ 'mismatch'::text) AND ((description)::text ~<~ 'mismatci'::text))
> Filter: ((description)::text ~~ 'mismatch%'::text)
> Total runtime: 0.919 ms
> (7 rows)
>
> Is there any tweaks to force pgsql to use index on description?
>
> Balaji
>
> P.S The event database has 700k records.
>

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83