Postgres Index a query with MAX and groupBy

Is there any way to index the following query?

SELECT run_id, MAX ( frame ) , MAX ( time ) FROM run.frames_stat GROUP BY run_id;

I’ve tried creating sorted (non-composite) indexes on frame and time, and an index on run_id, but the query planner doesn’t use them.

Misc info:

• Unfortunately (and for reasons I won’t get into) I cannot change the query
• The frames_stat table has 42 million rows
• The table is unchanging (no further inserts/deletes will ever take place)
• The query was always slow, it’s just gotten slower because this dataset is larger than in the past.
• There are no indexes on the table
• We are using Postgres 9.4
• The db’s “work_mem” size is 128MB (if that’s relevant).
• Hardware: 130GB Ram, 10 core Xeon

Schema:

CREATE TABLE run.frame_stat (
  id bigint NOT NULL,
  run_id bigint NOT NULL,
  frame bigint NOT NULL,
  heap_size bigint NOT NULL,
  "time" timestamp without time zone NOT NULL,
  CONSTRAINT frame_stat_pkey PRIMARY KEY (id)
)

Explain analyze:

HashAggregate  (cost=1086240.000..1086242.800 rows=280 width=24) (actual time=14182.426..14182.545 rows=280 loops=1)
  Group Key: run_id
  ->  Seq Scan on zulu  (cost=0.000..770880.000 rows=42048000 width=24) (actual time=0.037..4077.182 rows=42048000 loops=1)