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LIKE Query performance

From: "Hari, Balaji" <Balaji_Hari(at)bmc(dot)com>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: LIKE Query performance
Date: 2009-01-27 23:41:50
Message-ID: 425C04840926804AB81C297EDDE7B91717774F947E@HOUCCRPRD01.adprod.bmc.com (view raw or flat)
Thread:
Lists: pgsql-performance
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.

Responses

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Next:From: Oleg BartunovDate: 2009-01-28 06:26:54
Subject: Re: LIKE Query performance
Previous:From: Josh BerkusDate: 2009-01-27 17:33:47
Subject: Re: Odd behavior with temp usage logging

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