Still another try at fixing scanjoin_target insertion into parallel plans.

The previous code neglected the fact that the scanjoin_target might
carry sortgroupref labelings that we need to absorb.  Instead, do
create_projection_path() unconditionally, and tweak the path's cost
estimate after the fact.  (I'm now convinced that we ought to refactor
the way we account for sometimes not needing a separate projection step,
but right now is not the time for that sort of cleanup.)

Problem identified by Amit Kapila, patch by me.

src/test/regress/expected/select_parallel.out

@@ -6,9 +6,10 @@ create or replace function parallel_restricted(int) returns int as
 -- Serializable isolation would disable parallel query, so explicitly use an
 -- arbitrary other level.
 begin isolation level repeatable read;
--- setup parallel test
+-- encourage use of parallel plans
 set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
+set min_parallel_relation_size=0;
 set max_parallel_workers_per_gather=4;
 explain (costs off)
   select count(*) from a_star;
@@ -71,6 +72,21 @@ select length(stringu1) from tenk1 group by length(stringu1);
       6
 (1 row)
 
+explain (costs off)
+	select stringu1, count(*) from tenk1 group by stringu1 order by stringu1;
+                     QUERY PLAN                     
+----------------------------------------------------
+ Sort
+   Sort Key: stringu1
+   ->  Finalize HashAggregate
+         Group Key: stringu1
+         ->  Gather
+               Workers Planned: 4
+               ->  Partial HashAggregate
+                     Group Key: stringu1
+                     ->  Parallel Seq Scan on tenk1
+(9 rows)
+
 -- test that parallel plan for aggregates is not selected when
 -- target list contains parallel restricted clause.
 explain (costs off)

src/test/regress/sql/select_parallel.sql

@@ -9,9 +9,10 @@ create or replace function parallel_restricted(int) returns int as
 -- arbitrary other level.
 begin isolation level repeatable read;
 
--- setup parallel test
+-- encourage use of parallel plans
 set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
+set min_parallel_relation_size=0;
 set max_parallel_workers_per_gather=4;
 
 explain (costs off)
@@ -29,6 +30,9 @@ explain (costs off)
 	select length(stringu1) from tenk1 group by length(stringu1);
 select length(stringu1) from tenk1 group by length(stringu1);
 
+explain (costs off)
+	select stringu1, count(*) from tenk1 group by stringu1 order by stringu1;
+
 -- test that parallel plan for aggregates is not selected when
 -- target list contains parallel restricted clause.
 explain (costs off)