Re: [BUGS] LEFT JOINs not optimized away when not needed

On Tue, Jul 08, 2014 at 11:19:31AM -0400, Tom Lane wrote:
> Moshe Jacobson <moshe(at)neadwerx(dot)com> writes:
>
> > Seeing that there is only one output column, and that the results are
> > grouped by this output column, it seems to me that the optimizer should not
> > even look at the rest of the tables.
>
> The GROUP BY has nothing to do with it, but if all the other tables' join
> keys are primary keys (or at least unique), I'd expect the planner to get
> rid of the joins. However, I'm not sure whether it works completely when
> there are more than join_collapse_limit relations to worry about.

Eliminating JOINs seems orthogonal, at least in theory, to
join_collapse_limit. What have I missed here, and how might they have
dependencies?

Cheers,
David.
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