Does not use index on query using "field is null"?

2010/11/10 Bjørn T Johansen <btj@havleik.no>:

select * from table where field is null

And when I run explain, it tells me that it uses seq scan... Is this because pg thinks that seq scan is as fast as using indexes or because using index on "is null" queries does
not work?

Does the table have *lots* of NULLs in that column? If the index is
not very selective, it may choose to use a seq scan since it will
result in fewer I/O ops. This decision will be based on the relative
values of random_page_cost and seq_page_cost, and probably the
effective_cache_size as well.

=?ISO-8859-1?Q?Bj=F8rn?= T Johansen <btj@havleik.no> writes:

I have a query that looks like this...:
select * from table where field is null

And when I run explain, it tells me that it uses seq scan... Is this because pg thinks that seq scan is as fast as using indexes or because using index on "is null" queries does
not work?

What PG version? Releases before 8.3 do not think that IS NULL is an
indexable condition.

regards, tom lane

On Wed, 10 Nov 2010 10:00:43 -0500
Tom Lane <tgl@sss.pgh.pa.us> wrote:

=?ISO-8859-1?Q?Bj=F8rn?= T Johansen <btj@havleik.no> writes:

I have a query that looks like this...:
select * from table where field is null

And when I run explain, it tells me that it uses seq scan... Is this because pg thinks that seq scan is as fast as using indexes or because using index on "is null" queries does
not work?

What PG version? Releases before 8.3 do not think that IS NULL is an
indexable condition.

regards, tom lane

We are using version 8.4.x...

BTJ