Date: Tue, 11 Nov 2008 21:47:03 +0100
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: Using index for IS NULL query
Message-ID: <20081111204703.GA15745@tux>
References: <gfcprb$m40$1@news.hub.org>
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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
-- 
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