Subject: Bad performance on simple query
From: Dimi Paun <dimi@lattica.com>
To: pgsql-performance <pgsql-performance@postgresql.org>
Content-Type: text/plain
Organization: Lattica, Inc.
Date: Mon, 17 Nov 2008 10:53:17 -0500
Message-Id: <1226937197.5760.89.camel@dimi.lattica.com>
Mime-Version: 1.0
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Hi folks,

I have a simple table that keeps track of a user's access history.
It has a a few fields, but the important ones are:
  - ownerId: the user's ID, a int8
  - accessTS: the timestamp of the record

The table right now is small, only 1942 records.
The user I test with (10015) has only 89 entries.

What I want is to get the last 5 accesses of a user:
   SELECT * FROM triphistory WHERE ownerId = 10015 ORDER BY accessTS DESC LIMIT 5
 
If I create a composite index *and* analyze:
   create index IDX_TRIP_HISTORY_OWNER_ACCESS_TS on tripHistory (ownerId, accessTS);
   ANALYZE triphistory;

It takes 0.091s (!):
perpedes_db=# EXPLAIN ANALYZE SELECT * FROM triphistory WHERE ownerId = 10015 ORDER BY accessTS DESC LIMIT 5;
                                                                            QUERY PLAN                                                                            
------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..7.99 rows=5 width=106) (actual time=0.024..0.042 rows=5 loops=1)
   ->  Index Scan Backward using idx_trip_history_owner_access_ts on triphistory  (cost=0.00..142.20 rows=89 width=106) (actual time=0.021..0.034 rows=5 loops=1)
         Index Cond: (ownerid = 10015)
 Total runtime: 0.091 ms
(4 rows)


BTW, this is after several runs of the query, shouldn't all this stuff be in memory?

This is not a fast machine, but this seems rather excessive, no? 

-- 
Dimi Paun <dimi@lattica.com>
Lattica, Inc.