Re: Type conversions and nulls

On Tue, 11 May 2004, Edmund Dengler wrote:

> 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.

Use the full version or if you're generating the query strings on the fly
put only the check that matters (either the equality or the is null as
appropriate for the values). Or if all you care about is true/false,
possibly
select exists(select 1 from tmp where ...)
may be better.

Transform_null_equals is meant convert a very specific x = NULL or NULL =
x into x IS NULL. It doesn't (nor is it meant to) change how nulls
compare.