On Mon, Dec 28, 2009 at 2:32 PM, D. Dante Lorenso <dante@stripped> wrote:
>
> Will anything ever be equal to NULL in a SELECT query?
>
> SELECT *
> FROM sometable
> WHERE somecolumn = NULL;
>
> I have a real-life query like this:
>
> SELECT *
> FROM sometable
> WHERE somecolumn = NULL OR somecolumn = 'abc';
>
> The 'sometable' contains about 40 million records and in this query, it
> appears that the where clause is doing a sequential scan of the table to
> find a condition where 'somecolumn' = NULL. Shouldn't the query parser be
> smart enough to rewrite the above query like this:
>
> SELECT *
> FROM sometable
> WHERE FALSE OR somecolumn = 'abc';
>
> And therefor use the index I have on 'somecolumn'? When I manually rewrite
> the query, I get the performance I expect but when I leave it as it was,
> it's 100 times slower.
>
> What's so special about NULL?
http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html
Should answer some of your questions, Dante.
