postgres=# create table tbl(v text);
CREATE TABLE
postgres=# create table tbl_collate_c(v text collate "C");
CREATE TABLE
postgres=# explain select * from tbl order by v;
                                  QUERY PLAN
------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..431.00 rows=1 width=8)
   Merge Key: v
   ->  Sort  (cost=0.00..431.00 rows=1 width=8)
         Sort Key: v
         ->  Seq Scan on tbl  (cost=0.00..431.00 rows=1 width=8)
 Optimizer: Pivotal Optimizer (GPORCA)
(6 rows)

postgres=# explain select * from tbl_collate_c order by v;
                                      QUERY PLAN
---------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=1451.09..2199.09 rows=52800 width=32)
   Merge Key: v
   ->  Sort  (cost=1451.09..1495.09 rows=17600 width=32)
         Sort Key: v COLLATE "C"
         ->  Seq Scan on tbl_collate_c  (cost=0.00..210.00 rows=17600 width=32)
 Optimizer: Postgres query optimizer
(6 rows)