Как оптимизировать sql запрос (выборка из 1.5M строк)?

SELECT
  time AT TIME ZONE 'UTC' AT TIME ZONE 'MSK' AS time,
  type,
  CASE
    WHEN (duration > (extract(epoch from (time - '2015-08-29 12:36:50'))) )
    THEN extract(epoch from (time - '2015-08-29 12:36:50'))
    ELSE duration
  END AS trim_duration
FROM clamps
WHERE device_id = 27 AND time BETWEEN '2015-08-29 12:36:50' AND '2016-03-15 12:36:50'
ORDER BY time ASC

CREATE TABLE clamps
(
  id serial NOT NULL,
  "time" timestamp without time zone,
  duration numeric,
  type character varying(255),
  device_id integer,
  packet_id integer,
  dur_float double precision DEFAULT 0,
  created_at timestamp without time zone DEFAULT now(),
  updated_at timestamp without time zone DEFAULT now(),
  CONSTRAINT clamps_pkey PRIMARY KEY (id)
)

CREATE UNIQUE INDEX index_clamps_on_device_id_time
ON clamps
USING btree (device_id, "time");

Sort  (cost=149213.28..150770.01 rows=622695 width=21) (actual time=4263.385..4713.566 rows=1469514 loops=1)
  Sort Key: "time"
  Sort Method: external merge  Disk: 71816kB
  ->  Bitmap Heap Scan on clamps  (cost=17535.79..89284.59 rows=622695 width=21) (actual time=190.593..2692.161 rows=1469514 loops=1)
        Recheck Cond: ((device_id = 27) AND ("time" >= '2015-08-29 12:36:50'::timestamp without time zone) AND ("time" <= '2016-03-15 12:36:50'::timestamp without time zone))
        Heap Blocks: exact=17528
        ->  Bitmap Index Scan on index_clamps_on_device_id_time  (cost=0.00..17380.12 rows=622695 width=0) (actual time=186.667..186.667 rows=1469514 loops=1)
              Index Cond: ((device_id = 27) AND ("time" >= '2015-08-29 12:36:50'::timestamp without time zone) AND ("time" <= '2016-03-15 12:36:50'::timestamp without time zone))
Planning time: 0.291 ms
Execution time: 5414.691 ms