Patch for removng unused targets

Hi!

Attached patch removes unused targets which are used only for order by when data already comes in right order. It introduces resorderbyonly flag of TargetEntry which indicated that entry is used only for ORDER BY clause. If data comes in right order then such entries are removed in grouping_planner function.

This is my first patch on planner. Probably, I did it in wrong way. But I think it is worthwhile optimization and you could give me direction to rework patch.

Actually we meet need of this optimization when ranking full-text search in GIN index (it isn't published yet, will post prototype soon). But there is some synthetic example illustrating benefit from patch.

CREATE OR REPLACE FUNCTION slow_func(x float8, y float8) RETURNS float8 AS $$

BEGIN

PERFORM pg_sleep(0.01);

    RETURN x + y;

END;

$$ IMMUTABLE LANGUAGE plpgsql;

CREATE TABLE test AS (SELECT random() AS x, random() AS y FROM generate_series(1,1000));

CREATE INDEX test_idx ON test(slow_func(x,y));

Without patch:

test=# EXPLAIN (ANALYZE, VERBOSE) SELECT * FROM test ORDER BY slow_func(x,y) LIMIT 10;

                                                              QUERY PLAN                            

                                  

--------------------------------------------------------------------------------------------------------------------------------------

 Limit  (cost=0.00..3.09 rows=10 width=16) (actual time=11.344..103.443 rows=10 loops=1)

   Output: x, y, (slow_func(x, y))

   ->  Index Scan using test_idx on public.test  (cost=0.00..309.25 rows=1000 width=16) (actual time=11.341..103.422 rows=10 loops=1)

         Output: x, y, slow_func(x, y)

 Total runtime: 103.524 ms

(5 rows)

With patch:

test=# EXPLAIN (ANALYZE, VERBOSE) SELECT * FROM test ORDER BY slow_func(x,y) LIMIT 10;

                                                            QUERY PLAN                              

                               

-----------------------------------------------------------------------------------------------------------------------------------

 Limit  (cost=0.00..3.09 rows=10 width=16) (actual time=0.062..0.093 rows=10 loops=1)

   Output: x, y

   ->  Index Scan using test_idx on public.test  (cost=0.00..309.25 rows=1000 width=16) (actual time=0.058..0.085 rows=10 loops=1)

         Output: x, y

 Total runtime: 0.164 ms

(5 rows)