Re: Todo: Teach planner to evaluate multiple windows in the optimal order

Hi David,

Please find attached patch with addressed issues mentioned before.

Things resolved:

1. Correct position of window function from where order by push down can 
happen

is determined, this fixes issue mentioned in case #1.

> While writing test cases, I found that optimization do not happen for
> case #1
>
> (which is prime candidate for such operation) like
>
> EXPLAIN (COSTS OFF)
> SELECT empno,
>         depname,
>         min(salary) OVER (PARTITION BY depname ORDER BY empno) depminsalary,
>         sum(salary) OVER (PARTITION BY depname) depsalary
> FROM empsalary
> ORDER BY depname, empno, enroll_date

2. Point #2 as in above discussions

> a) looks like the best plan to me.  What's the point of pushing the
> sort below the WindowAgg in this case? The point of this optimisation
> is to reduce the number of sorts not to push them as deep into the
> plan as possible. We should only be pushing them down when it can
> reduce the number of sorts. There's no reduction in the number of
> sorts in the above plan.

Works as mentioned.

All test cases (newly added and existing ones) are green.

-- 
Regards,
Ankit Kumar Pandey