From: anthony.shipman@symstream.com
To: tv@fuzzy.cz
Cc: pgsql-performance@postgresql.org, "Claudio Freire" <klaussfreire@gmail.com>
Cc: Pavel Stehule <pavel.stehule@gmail.com>
Subject: Re: strange query plan with LIMIT
Date: Fri, 10 Jun 2011 18:38:39 +1000
Message-ID: <201106101838.39781.anthony.shipman@symstream.com> (raw)
In-Reply-To: <c48d12d868ecc4b9ab6063f562650722.squirrel@sq.gransy.com>
References: <201106071802.08272.anthony.shipman@symstream.com>
<201106081834.07263.anthony.shipman@symstream.com>
<c48d12d868ecc4b9ab6063f562650722.squirrel@sq.gransy.com>
On Wednesday 08 June 2011 19:47, tv@fuzzy.cz wrote:
> Have you tried to create a composite index on those two columns? Not sure
> if that helps but I'd try that.
>
> Tomas
This finally works well enough
CREATE TABLE tdiag (
diag_id integer DEFAULT nextval('diag_id_seq'::text),
create_time timestamp with time zone default now(),
....
PRIMARY KEY (diag_id)
);
-- ************ COMPOSITE INDEX
create index tdiag_id_create on tdiag(diag_id, create_time);
alter table tdiag alter column diag_id set statistics 1000;
alter table tdiag alter column create_time set statistics 1000;
and then just do the original query
symstream2=> explain analyze select * from tdiag where
symstream2-> (create_time >= '2011-06-07 02:00:00.000000+0' and create_time
< '2011-06-10 07:58:03.000000+0') and diag_level <= 1
symstream2-> order by diag_id LIMIT 100 OFFSET 800;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=6064.19..6822.21 rows=100 width=112) (actual
time=1496.644..1497.094 rows=100 loops=1)
-> Index Scan using tdiag_id_create on tdiag (cost=0.00..1320219.58
rows=174166 width=112) (actual time=1409.285..1495.831 rows=900 loops=1)
Index Cond: ((create_time >= '2011-06-07 12:00:00+10'::timestamp with
time zone) AND (create_time < '2011-06-10 17:58:03+10'::timestamp with time
zone))
Filter: (diag_level <= 1)
Total runtime: 1497.297 ms
If I had set the primary key to (diag_id, create_time) would simple queries on
diag_id still work well i.e.
select * from tdiag where diag_id = 1234;
--
Anthony Shipman | -module(erlang).
Anthony.Shipman@symstream.com | ''(_)->0. %-)
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To: pgsql-performance@postgresql.org
Cc: anthony.shipman@symstream.com, tv@fuzzy.cz, klaussfreire@gmail.com, pavel.stehule@gmail.com
Subject: Re: strange query plan with LIMIT
In-Reply-To: <201106101838.39781.anthony.shipman@symstream.com>
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