Re: Avoid extra Sort nodes between WindowAggs when sorting can bereused

Daniel,

I took a look at the patch. It applies and compiles, the tests pass.

Some thoughts about the code:

* Postgres lists cache their lengths, so you don't need uniqueLen.

* Use an array of WindowClauseSortNode's instead of a list. It's more 
suitable here because you are going to sort (qsort_list creates a 
temporary array).

* Reversing the sorted list looks more confusing to me than just sorting 
it in the proper order right away. A qsort() comparator returning 
negative means "left goes before right", but here it is effectively the 
other way around.

* This isn't relevant given the previous points, but to reverse a list, 
you can walk it with foreach() and construct a reversed version with 
lcons().

* There is a function named make_pathkeys_for_window that makes a list 
of canonical pathkeys given a window clause. Using this function to give 
you the pathkeys, and then comparing them, would be more future-proof in 
case we ever start using hashing for windowing. Moreover, the canonical 
pathkeys can be compared with pointer comparison which is much faster 
than equal(). Still, I'm not sure whether it's going to be convenient to 
use in this case, or even whether it is a right thing to do. What do you 
think?

-- 
Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company