From: Dimi Paun <dimi@lattica.com>
To: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: How to use indexes for GROUP BY
Date: Mon, 24 Jan 2011 13:29:01 -0500
Message-ID: <1295893741.9380.984.camel@dimi.lattica.com> (raw)
Hi folks,
I have a table like so:
create table tagRecord (
uid varchar(60) primary key,
[bunch of other fields]
location varchar(32),
creationTS timestamp
);
create index idx_tagdata_loc_creationTS on tagRecord(location, creationTS);
The number of individual values in location is small (e.g. 2).
I want to simply get the latest "creationTS" for each location,
but that seems to result in a full table scan:
tts_server_db=# explain analyze select location, max(creationTS) from tagrecord group by location;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=5330.53..5330.55 rows=2 width=18) (actual time=286.161..286.165 rows=3 loops=1)
-> Seq Scan on tagrecord (cost=0.00..4771.35 rows=111835 width=18) (actual time=0.059..119.828 rows=111739 loops=1)
Total runtime: 286.222 ms
Now I have the idx_tagdata_loc_creationTS, and it seemed to me that
it should be able to use it to quickly figure out the max creationTS
for each location.
Any way I can make this more efficient?
BTW, I am using postgresql-server-8.1.22-1.el5_5.1
--
Dimi Paun <dimi@lattica.com>
Lattica, Inc.
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To: pgsql-performance@postgresql.org
Cc: dimi@lattica.com
Subject: Re: How to use indexes for GROUP BY
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