Performance issue with NestedLoop query

Hi,

I am trying to see if I can do anything to optimize the following plan.

I have two tables and I am doing a join between them. After joining it
calculates aggregates (Sum and Count)
Table 1 : timestamp (one per day) for 2 years (730 records)
Table 2 : Window based validity records. Window here means start and end
timestamp indicating a period of validity for a record.
Hash some 10 odd columns including start_time and end_time. (1 million
records)

Machine has 244 GB RAM. Queries are taking more than a min and in some case
2-3 mins.

Below is the plan I am getting. The Nested loop blows up the number of
records and we expect that. I have tried playing around work_mem and cache
configs which hasn't helped.

Query
select sum(a), count(id), a.ts, st from table1 a, table2 b where a.ts >
b.start_date and a.ts < b.end_date and a.ts > '2015-01-01 20:50:44.000000
+00:00:00' and a.ts < '2015-07-01 19:50:44.000000 +00:00:00' group by a.ts,
st order by a.ts

Plan (EXPLAIN ANALYZE)
"Sort (cost=10005447874.54..10005447879.07 rows=1810 width=44) (actual
time=178883.936..178884.159 rows=1355 loops=1)"
" Output: (sum(b.a)), (count(b.id)), a.ts, b.st"
" Sort Key: a.ts"
" Sort Method: quicksort Memory: 154kB"
" Buffers: shared hit=47068722 read=102781"
" I/O Timings: read=579.946"
" -> HashAggregate (cost=10005447758.51..10005447776.61 rows=1810
width=44) (actual time=178882.874..178883.320 rows=1355 loops=1)"
" Output: sum(b.a), count(b.id), a.ts, b.st"
" Group Key: a.ts, b.st"
" Buffers: shared hit=47068719 read=102781"
" I/O Timings: read=579.946"
" -> Nested Loop (cost=10000000000.43..10004821800.38
rows=62595813 width=44) (actual time=0.167..139484.854 rows=73112419
loops=1)"
" Output: a.ts, b.st, b.a, b.id"
" Buffers: shared hit=47068719 read=102781"
" I/O Timings: read=579.946"
" -> Seq Scan on public.table1 a (cost=0.00..14.81 rows=181
width=8) (actual time=0.058..0.563 rows=181 loops=1)"
" Output: a.ts"
" Filter: ((a.ts > '2015-01-01 20:50:44+00'::timestamp
with time zone) AND (a.ts < '2015-07-01 19:50:44+00'::timestamp with time
zone))"
" Rows Removed by Filter: 540"
" Buffers: shared read=4"
" I/O Timings: read=0.061"
" -> Index Scan using end_date_idx on public.table2 b
(cost=0.43..23181.37 rows=345833 width=52) (actual time=0.063..622.274
rows=403936 loops=181)"
" Output: b.serial_no, b.name, b.st, b.end_date, b.a,
b.start_date"
" Index Cond: (a.ts < b.end_date)"
" Filter: (a.ts > b.start_date)"
" Rows Removed by Filter: 392642"
" Buffers: shared hit=47068719 read=102777"
" I/O Timings: read=579.885"
"Planning time: 0.198 ms"
"Execution time: 178884.467 ms"

Any pointers on how to go about optimizing this?

--yr