GroupAggregate and Integer Arrays

Hi,

Wondering if anyone could suggest how we could improve the performance of
this type of query?
The intensive part is the summing of integer arrays as far as I can see.
We're thinking there's not much we can do to improve performance apart from
throw more CPU at it... would love to be proven wrong though!

*Query:*

explain (analyse,buffers)
select
sum(s2.array_a),sum(s2.array_b)
from mytable s1 left join mytable s2
on s1.code=s2.code and s1.buyer=s2.seller and s2.seller='XX'
where s1.buyer='XX'
group by s1.buyer,s1.code
;

*Depesz Explain Link:*

http://explain.depesz.com/s/m3XP

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=275573.49..336223.36 rows=2547 width=524) (actual
time=1059.340..22946.772 rows=22730 loops=1)
Buffers: shared hit=113596 read=1020 dirtied=15
-> Merge Left Join (cost=275573.49..278850.09 rows=113560 width=524)
(actual time=1058.773..1728.186 rows=240979 loops=1)
Merge Cond: ((s1.code)::text = (s2.code)::text)
Join Filter: (s1.buyer = (s2.seller)::bpchar)
Buffers: shared hit=113596 read=1020 dirtied=15
-> Index Only Scan using mytable_buyer_idx on mytable s1
(cost=0.42..1226.06 rows=25465 width=12) (actual time=0.015..35.790
rows=22730 loops=1)
Index Cond: (buyer = 'XX'::bpchar)
Heap Fetches: 3739
Buffers: shared hit=16805 dirtied=1
-> Sort (cost=275573.07..275818.33 rows=98106 width=525) (actual
time=1058.736..1141.560 rows=231662 loops=1)
Sort Key: s2.code
Sort Method: quicksort Memory: 241426kB
Buffers: shared hit=96791 read=1020 dirtied=14
-> Bitmap Heap Scan on mytable s2
(cost=12256.28..267439.07 rows=98106 width=525) (actual
time=60.330..325.730 rows=231662 loops=1)
Recheck Cond: ((seller)::text = 'XX'::text)
Filter: ((seller)::bpchar = 'XX'::bpchar)
Buffers: shared hit=96791 read=1020 dirtied=14
-> Bitmap Index Scan on mytable_seller_idx
(cost=0.00..12231.75 rows=254844 width=0) (actual time=40.474..40.474
rows=233244 loops=1)
Index Cond: ((seller)::text = 'XX'::text)
Buffers: shared hit=30 read=1020
Total runtime: 22968.292 ms
(22 rows)

*Table size:*

=> select count(*) from mytable;
count
--------
602669
(1 row)

*Array types:*

# select array_a,array_b from mytable limit 1;
array_a | array_b
---------------------------+---------------------------
{0,0,0,0,0,0,0,0,0,0,0,0} | {0,0,0,0,0,0,0,0,0,0,0,0}

*Example schema:*

# \d mytable
Table "public.mytable"
Column | Type | Modifiers
-------------------+-----------------------+------------------------
buyer | character(2) | not null
code | character varying(20) | not null
seller | character varying(50) |
array_a | integer[] |
array_b | integer[] |
Indexes:
"mytable_buyer_code_idx" UNIQUE, btree (buyer, code) CLUSTER
"mytable_buyer_idx" btree (buyer)
"mytable_code_idx" btree (code)
"mytable_seller_idx" btree (seller)

*Version:*

> SELECT version() ;
version

--------------------------------------------------------------------------------------------------------------
PostgreSQL 9.3.6 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.6.3
20120306 (Red Hat 4.6.3-2), 64-bit
(1 row)

This is running on an AWS RDS instance.

Thanks for any pointers
--
David