Re: Index on a NULL-value

Tobias Brox <tobias@nordicbet.com> writes:

> [Bruno Wolff III - Mon at 10:36:33PM -0500]
> > You want something like this:
> > CREATE INDEX b_is_null ON mock(b) WHERE b IS NULL;
>
> Oh, cool.  I wasn't aware that this is possible.  This would probably help
> us a lot of places. :-)

Yeah it's a cool feature.

I'm not 100% sure but I think it still won't consider this index unless the
column being indexed is used in some indexable operation. So for example if
you had

CREATE INDEX b_null on mock(other) WHERE b IS NULL;

and something like

 SELECT * FROM b WHERE b IS NULL ORDER BY other
or
 SELECT * FROM b where other > 0 AND b IS NULL

then it would be a candidate because the ORDER BY or the other > 0 make the
index look relevant. But I don't think (again I'm not 100% sure) that the
partial index WHERE clause is considered in picking which indexes to consider.

It *is* considered in evaluating which index is the best one to use and
whether it's better than a sequential scan. Just not in the initial choice of
which indexes to look at at all.

--
greg