Btree vs trigram index for short prefixes

gistfile1.txt

filesys=# \d file
                                    Table "public.file"
  Column  |            Type             |                     Modifiers
----------+-----------------------------+---------------------------------------------------
 id       | integer                     | not null default nextval('file_id_seq'::regclass)
 name     | character varying           | not null
 size     | bigint                      | not null
 modified | timestamp without time zone |
 md5      | character(32)               |
Indexes:
    "file_ix_btree_name" btree (name)
    "file_ix_name" gist (name gist_trgm_ops) INVALID



Using the btree index (straight after creating the index):

filesys=# explain (analyze, buffers) select * from file where name like '/%';
                                                           QUERY PLAN

------------------------------------------------------------------------------------------------------------------------
---------
 Bitmap Heap Scan on file  (cost=546.39..37279.47 rows=404 width=72) (actual time=0.021..0.021 rows=0 loops=1)
   Filter: ((name)::text ~~ '/%'::text)
   Buffers: shared hit=3 read=1
   ->  Bitmap Index Scan on file_ix_btree_name  (cost=0.00..546.29 rows=19923 width=0) (actual time=0.018..0.018 rows=0
loops=1)
         Index Cond: (((name)::text >= '/'::text) AND ((name)::text < '0'::text))
         Buffers: shared hit=3 read=1
 Planning time: 0.543 ms
 Execution time: 0.039 ms
(8 rows)


Using the trigram index (straight after reindexing):

filesys=# explain (analyze, buffers) select * from file where name like '/%';
                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------
------------
 Bitmap Heap Scan on file  (cost=24.55..1356.08 rows=404 width=72) (actual time=2371.035..2371.035 rows=0 loops=1)
   Recheck Cond: ((name)::text ~~ '/%'::text)
   Rows Removed by Index Recheck: 4204725
   Heap Blocks: exact=57608
   Buffers: shared hit=27869 read=90491 written=883
   ->  Bitmap Index Scan on file_ix_name  (cost=0.00..24.45 rows=404 width=0) (actual time=1198.344..1198.344 rows=42047
25 loops=1)
         Index Cond: ((name)::text ~~ '/%'::text)
         Buffers: shared hit=27868 read=32884
 Planning time: 0.317 ms
 Execution time: 2371.490 ms
(10 rows)