Re: Patch for removng unused targets

 Don't forget about window functions!

test=# EXPLAIN (ANALYZE, VERBOSE) SELECT *, count(*) over (partition by slow_func(x,y)) FROM test ORDER BY slow_func(x,y) LIMIT 10;                                                                QUERY PLAN                                                                 ------------------------------------------------------------------------------------------------------------------------------------------- Limit  (cost=0.28..3.52 rows=10 width=16) (actual time=20.860..113.764 rows=10 loops=1)
   Output: x, y, (count(*) OVER (?))
   ->  WindowAgg  (cost=0.28..324.27 rows=1000 width=16) (actual time=20.858..113.747 rows=10 loops=1)
         Output: x, y, count(*) OVER (?)
         ->  Index Scan using test_idx on public.test  (cost=0.28..59.27 rows=1000 width=16) (actual time=10.563..113.530 rows=11 loops=1)
               Output: slow_func(x, y), x, y
 Total runtime: 117.889 ms
(7 rows)

And I don't think it's a good idea to rely on the parse tree to see if we can remove those unused columns from the target list, because there should be a lot of optimization that has been done through grouping_planner, and the parse tree is not necessarily representing the corresponding elements at this point.  I think it'd be better to see path keys to find out the list of elements that may be removed, rather than SortClause, which would be a more generalized approach.

Thanks,