Re: slow information schema with thausand users, seq.scan pg_authid

Andrew - Supernews <andrew+nonews(at)supernews(dot)com> writes:
> On 2006-02-06, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:
>> It already has indexes.

> True, but they're not being used where you'd expect. This seems to be
> something to do with the fact that it's not pg_authid which is being
> accessed, but rather the view pg_roles.

I looked into this and it seems the problem is that the view doesn't
get flattened into the main query because of the has_nullable_targetlist
limitation in prepjointree.c. That's triggered because pg_roles has
'********'::text AS rolpassword
which isn't nullable, meaning it would produce wrong behavior if
referenced above the outer join.

Ultimately, the reason this is a problem is that the planner deals only
in simple Vars while processing joins; it doesn't want to think about
expressions. I'm starting to think that it may be time to fix this,
because I've run into several related restrictions lately, but it seems
like a nontrivial project.

In the meantime, reducing the LEFT JOIN to pg_roles to a JOIN as per
Peter's suggestion seems like the best short-term workaround.

regards, tom lane