Type conversions and nulls

Howdy all!

Just checking on whether this is the expected behaviour. I am transferring
data from multiple databases to single one, and I want to ensure that I
only have unique rows for some tables. Unfortunately, some of the rows
have nulls for various columns, and I want to compare them for exact
equality.

=> create table tmp (
bigint a,
bigint b,
primary key (a, b)
);

To test for existence, I would naively use:

=> select count(1) from tmp
where a = <value>
and b = <value>;

What I should use is:

=> select count(1) from tmp
where ((a = <value>) or (a is null and <value> is null))
and ((b = <value>) or (b is null and <value> is null));

Looking in the manual, I see I can get what I want by running:

=> set transform_null_equals to on;

And I can go back to using my naive script and everything works.

However, as <values> are integers, I need to convert them to bigint's so
that the index can be used (Postgresql 7.4.2 automatic casts, unless this
has been fixed). So I wrote my script to do the following

=> select count(1) from tmp
where a = <value>::bigint
and b = <value>::bigint;

And now the nulls don't match! As a further test, I did:

=> select null = null, null = null::bigint, null::bigint = null::bigint;
?column? | ?column? | ?column?
----------+----------+----------
t | t |
(1 row)

So, is there a way to do the casts such that this works? Other
alternatives? I did a search but couldn't find an answer on the archives.

Regards!
Ed