From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: Using index for IS NULL query
Date: Tue, 11 Nov 2008 21:47:03 +0100
Message-ID: <20081111204703.GA15745@tux> (raw)
In-Reply-To: <gfcprb$m40$1@news.hub.org>
References: <gfcprb$m40$1@news.hub.org>
Andrus <kobruleht2@hot.ee> schrieb:
> Index is not used for
>
> is null
>
> condition:
>
> create index makse_dokumnr_idx on makse(dokumnr);
> explain select
> sum( summa)
> from MAKSE
> where dokumnr is null
>
> "Aggregate (cost=131927.95..131927.96 rows=1 width=10)"
> " -> Seq Scan on makse (cost=0.00..131927.94 rows=1 width=10)"
> " Filter: (dokumnr IS NULL)"
>
>
>
> Table makse contains 1200000 rows and about 800 rows with dokumnr is null
> so using index is much faster that seq scan.
> How to fix ?
Create a partial index like below:
test=# create table foo ( i float);
CREATE TABLE
Zeit: 1,138 ms
test=*# insert into foo select random() from generate_series(1,1000000);
INSERT 0 1000000
test=*# insert into foo values (NULL);
INSERT 0 1
test=*# create index idx_foo on foo(i) where i is null;
CREATE INDEX
test=*# explain analyse select * from foo where i is null;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on foo (cost=5.51..4690.89 rows=5000 width=8) (actual
time=0.037..0.038 rows=1 loops=1)
Recheck Cond: (i IS NULL)
-> Bitmap Index Scan on idx_foo (cost=0.00..4.26 rows=5000 width=0)
(actual time=0.033..0.033 rows=1 loops=1)
Index Cond: (i IS NULL)
Total runtime: 0.068 ms
(5 Zeilen)
Maybe there are other solutions...
Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect. (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly." (unknown)
Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: akretschmer@spamfence.net
Subject: Re: Using index for IS NULL query
In-Reply-To: <20081111204703.GA15745@tux>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox