Re: pgsql: Further fixes for degenerate outer join clauses.

On 08/06/2015 03:36 PM, Tom Lane wrote:
> Further fixes for degenerate outer join clauses.
>
> Further testing revealed that commit f69b4b9495269cc4 was still a few
> bricks shy of a load: minor tweaking of the previous test cases resulted
> in the same wrong-outer-join-order problem coming back.  After study
> I concluded that my previous changes in make_outerjoininfo() were just
> accidentally masking the problem, and should be reverted in favor of
> forcing syntactic join order whenever an upper outer join's predicate
> doesn't mention a lower outer join's LHS.  This still allows the
> chained-outer-joins style that is the normally optimizable case.
>
> I also tightened things up some more in join_is_legal().  It seems to me
> on review that what's really happening in the exception case where we
> ignore a mismatched special join is that we're allowing the proposed join
> to associate into the RHS of the outer join we're comparing it to.  As
> such, we should *always* insist that the proposed join be a left join,
> which eliminates a bunch of rather dubious argumentation.  The case where
> we weren't enforcing that was the one that was already known buggy anyway
> (it had a violatable Assert before the aforesaid commit) so it hardly
> deserves a lot of deference.
>
> Back-patch to all active branches, like the previous patch.  The added
> regression test case failed in all branches back to 9.1, and I think it's
> only an unrelated change in costing calculations that kept 9.0 from
> choosing a broken plan.


Looks like this might have upset brolga on 9.0 and 9.1 - it's coming up
with a different plan from what's expected.


cheers

andrew