Index could be added on product_price_history

Going through a performance / index analysis, I used this query (from http://stackoverflow.com/questions/3318727/postgresql-index-usage-analysis):

SELECT relname, seq_scan-idx_scan AS too_much_seq, case when seq_scan-idx_scan>0 THEN 'Missing Index?' ELSE 'OK' END, pg_relation_size(relname::regclass) AS rel_size, seq_scan, idx_scan
 FROM pg_stat_all_tables
 WHERE schemaname='public' AND pg_relation_size(relname::regclass)>80000 ORDER BY too_much_seq DESC;

And it appeared that product_price_history is always accessed using a seq scan.

I also noticed in the logs that the SELECT queries on this table is always in this form:

SELECT DISTINCT ON (product_id, name) datetime, product_id, name, amount FROM product_price_history WHERE product_id IN (5521) AND company_id =
 3 AND name IN ('standard_price') AND datetime <= '2014-05-03 09:34:30' ORDER BY product_id, name, datetime DESC, id DESC;

So, it could be nice to add an index on the 4 fields.

Explain plan before:

                                                                                    QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   Unique (cost=329.59..329.60 rows=1 width=32) (actual time=4.858..4.861 rows=1 loops=1)
   -> Sort (cost=329.59..329.60 rows=1 width=32) (actual time=4.854..4.855 rows=1 loops=1)
         Sort Key: datetime, id
         Sort Method: quicksort Memory: 25kB
         -> Seq Scan on product_price_history (cost=0.00..329.58 rows=1 width=32) (actual time=0.020..4.779 rows=1 loops=1)
               Filter: ((datetime <= '2014-05-03 09:34:30'::timestamp without time zone) AND (product_id = 5521) AND ((name)::text = 'standard_price'::text) AND (company_id = 3))
 Total runtime: 4.905 ms

Explain plan after index
create index product_price_history_all_index on product_price_history (product_id, company_id, name, datetime);

                                                                                      QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   Unique (cost=8.29..8.30 rows=1 width=32) (actual time=0.135..0.136 rows=1 loops=1)
   -> Sort (cost=8.29..8.29 rows=1 width=32) (actual time=0.133..0.134 rows=1 loops=1)
         Sort Key: datetime, id
         Sort Method: quicksort Memory: 25kB
         -> Index Scan using product_price_history_all_index on product_price_history (cost=0.00..8.28 rows=1 width=32) (actual time=0.117..0.119 rows=1 loops=1)
               Index Cond: ((product_id = 5521) AND (company_id = 3) AND ((name)::text = 'standard_price'::text) AND (datetime <= '2014-05-03 09:34:30'::timestamp without time zone))
 Total runtime: 0.190 ms