Remove LEFT part of JOIN to pg_roles because of optimizer limitation:

> 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.

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