Re: A more efficient way?

James Cloos <cloos(at)jhcloos(dot)com> writes:
> I've a third-party app which is slower than it ought to be.
> It does one operation in particular by doing a query to get a list of
> rows, and then iterates though them to confirm whether it actually wants
> that row. As you can imagine that is very slow.

> This query gets the correct data set in one go, but is also slow:

> select p from m where s = 7 and p not in (select p from m where s != 7);

See if you can recast it as a NOT EXISTS. NOT IN is hard to optimize
because of its weird behavior with nulls.

If you're working with a less-than-current version of PG, you may
instead have to resort to a left-join-with-is-null locution, ie
select m.p from m left join
(select p from m where s != 7) m2
on (m.p=m2.p)
where m2.p is null and m.s = 7;
but it's hard to wrap your head around that sort of thing, so I'd
advise against using it if you can get decent performance with EXISTS.

regards, tom lane