Re: why "WHERE uid NOT IN" is so slow, and EXCEPT in the same situtation is fast?

On Wed, Jul 30, 2008 at 11:08:06PM -0400, Tom Lane wrote:
> Hmm, what have you got work_mem set to?  The first one would likely
> have been a lot faster if it had hashed the subplan; which I'd have
> thought would happen with only 80K rows in the subplan result,
> except it didn't.

work_mem = 1024kB

The machine has 48 MB total RAM and is a Xen host.

> The queries are in fact not exactly equivalent, because EXCEPT
> involves some duplicate-elimination behavior that won't happen
> in the NOT IN formulation.  So I don't apologize for your having
> gotten different plans.

But if use EXCEPT ALL?

> Another issue is that the NOT IN will probably not do what you
> expected if the subquery yields any NULLs.

In this specific query I think it is not possible for the subquery to
have NULLs, because its an INNER JOIN USING (the_only_column_in_the
_result, some_other_column_also). If any "uid" column of any row would
have been NULL, it wouldn't appear in that INNER JOIN, no?

--
Miernik
http://miernik.name/