Ignoring index on (A is null), (A is not null) conditions

Hi,

suppose, for simplicity, there is a table with index like this:

create table TABLE1 (
A integer
);
create index TABLE1_A on TABLE1 (A);

My question is: why psql (7.3.3) does not use index when filtering by A IS
NULL, A IS NOT
NULL expressions?

In fact, I need to filter by expression ((A is null) or (A > const)).

Is there a way to filter by this expression using index?

Functional index cannot be used (except strange solution with CASE-ing and
converting NULL values into some integer constant)

----------------------------------------------------------------------------
--
Index Scan using table1_a on table1 (cost=0.00..437.14 rows=29164 width=4)
Index Cond: (a > 1000)
----------------------------------------------------------------------------
--
Seq Scan on table1 (cost=0.00..448.22 rows=1 width=4)
Filter: (a IS NULL)
--------------------------------------------------------
Seq Scan on table1 (cost=0.00..448.22 rows=30222 width=4)
Filter: (a IS NOT NULL)
------------------------------------------------------------
Seq Scan on table1 (cost=0.00..523.77 rows=29164 width=4)
Filter: ((a IS NULL) OR (a > 1000))
------------------------------------------------------------

CH