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Re: Index on a NULL-value

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Tobias Brox <tobias(at)nordicbet(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Index on a NULL-value
Date: 2005-05-31 03:16:53
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Lists: pgsql-performance
On Tue, May 31, 2005 at 11:02:07 +0800,
  Tobias Brox <tobias(at)nordicbet(dot)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);
> test=# create index b_is_null on mock((b IS NULL));
> 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; 
> 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.

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