Date: Mon, 30 May 2005 22:16:53 -0500
From: Bruno Wolff III <bruno@wolff.to>
To: Tobias Brox <tobias@nordicbet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Index on a NULL-value
Message-ID: <20050531031653.GA17723@wolff.to>
Mail-Followup-To: Bruno Wolff III <bruno@wolff.to>,
	Tobias Brox <tobias@nordicbet.com>, pgsql-performance@postgresql.org
References: <20050531030207.GD26100@tobias.exoweb.net>
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On Tue, May 31, 2005 at 11:02:07 +0800,
  Tobias Brox <tobias@nordicbet.com> wrote:
> I read in the manual today:
> 
>   Indexes are not used for IS NULL clauses by default. The best way to use
>   indexes in such cases is to create a partial index using an IS NULL
>   predicate.
>   
> This is from the documentation for PostgreSQL 8.  I did not find anything
> equivalent in the 7.4.8-documentation.
>   
> I wasn't aware of this until it became an issue :-) Well, so I follow the
> tip but in vain.  Reduced and reproduced like this in PostgreSQL 7.4.7:
> 
> test=# create table mock(a int, b int);
> CREATE TABLE
> test=# create index b_is_null on mock((b IS NULL));
> CREATE INDEX
> test=# insert into mock values (10,20);
> INSERT 70385040 1
> test=# insert into mock values (20,30);
> INSERT 70385041 1
> test=# insert into mock values (30, NULL);
> INSERT 70385042 1
> test=# set enable_seqscan=off; 
> SET
> test=# explain select * from mock where b is NULL;
>                              QUERY PLAN                             
> --------------------------------------------------------------------
>  Seq Scan on mock (cost=100000000.00..100000020.00 rows=6 width=8)
>    Filter: (b IS NULL)
> (2 rows)
> 
> vacuum analyze also didn't help to recognize the index ;-)

It isn't surprising that an index wasn't used since a sequential scan is
going to be faster in your test case.

If you want to test this out, you to want use realistically sized tables.