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由 Bruce Momjian 提交于
> 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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