Re: Optimising queries involving unions

Sam Mason <sam@samason.me.uk> writes:
> Here's a bad example:

>   SELECT u.txt
>   FROM smalltable t, (
>     SELECT id, txt FROM largetable1
>     UNION ALL
>     SELECT id, txt FROM largetable2) u
>   WHERE t.id = u.id
>     AND t.foo = 'bar';

> I was hoping that "smalltable" would get moved up into the union,
> but it doesn't at the moment and the database does a LOT of extra
> work.

I'm afraid we're a long way away from being able to do that; the
parse/plan representation of UNION wasn't chosen with an eye to
being able to optimize it at all :-(.  We can push restriction
clauses down into a union, but we can't do much with join clauses,
because they necessarily refer to tables that don't even exist
within the sub-query formed by the UNION.

It'd be nice to fix this someday, but don't hold your breath ...

            regards, tom lane