On Mon, 2005-10-31 at 13:13, Merlin Moncure wrote:
> > > if that index is causing the problem, you may want to consider
> setting
> > > up partial index to exclude null values.
> >
> > This is a single column index. I assumed that null column values were
> > not indexed. Is my assumption incorrect?
> >
> > -K
> It turns out it is, or it certainly seems to be. I didn't know that :).
> So partial index will probably not help for null exclusion...
>
> would be interesting to see if you are getting swaps (check pg_tmp) when
> performance breaks down. That is an easy fix, bump work_mem.
OK, here's the issue in a nutshell.
NULLS, like everything else, are indexed. HOWEVER, there's no way for
them to be used by a normal query, since =NULL is not a legal
construct. So, you can't do something like:
select * from sometable where somefield = NULL
because you won't get any answers, since nothing can equal NULL and
select * from sometable where somefield IS NULL won't work because IS is
not a nomally indexible operator.
Which is why you can create two indexes on a table to get around this
like so:
create index iname1 on table (field) where field IS NULL
and
create index iname2 on table (field) where field IS NOT NULL
And then the nulls are indexable by IS / IS NOT NULL.
