slow query - will CLUSTER help?

Hello,_
_
I've got a very simple table with a very simple SELECT query, but it
takes longer on the initial run than I'd like, so I want to see if there
is a strategy to optimize this.

Table rt_h_nbbo contains several hundred million rows. All rows for a
given entry_date are appended to this table in an overnight process
every night - on the order of several million rows per day.

The objective is to select all of the rows for a given product_id on a
given entry_date.

There is a b-tree index on (product_id, entry_date). The index appears
to be used correctly. I'm seeing that if the data pages are not in
memory, nearly all of the time is spent on disk I/O. The first time,
the query takes 21 sec. If I run this query a second time, it completes
in approx 1-2 ms.

I perceive an inefficiency here and I'd like your input as to how to
deal with it: The end result of the query is 1631 rows which is on the
order of about a couple hundred Kb of data. Compare that to the amount
of I/O that was done: 1634 buffers were loaded, 16Mb per page - that's
about 24 Gb of data! Query completed in 21 sec. I'd like to be able to
physically re-organize the data on disk so that the data for a given
product_id on a entry_date is concentrated on a few pages instead of
being scattered like I see here.

_First question_ is: Does loading 24Gb of data in 21 sec seem "about
right" (hardware specs at bottom of email)?

_Second question_: Is it possible to tell postgres to physically store
the data in such a way that it parallels an index? I recall you can do
this in Sybase with a CLUSTERED index. The answer for Postgresql seems
to be "yes, use the CLUSTER command". But this command does a one-time
clustering and requires periodic re-clustering. Is this the best
approach? Are there considerations with respect to the type of index
(B-tree, GIST, SP-GIST) being used for CLUSTER ?

Thanks

-Sev

_Table (this is a fairly large table - hundreds of millions of rows):_

CREATE TABLE rt_h_nbbo
(
product_id integer NOT NULL,
bid_price double precision NOT NULL DEFAULT 0.0,
bid_size integer NOT NULL DEFAULT 0,
ask_price double precision NOT NULL DEFAULT 0.0,
ask_size integer NOT NULL DEFAULT 0,
last_price double precision NOT NULL DEFAULT 0.0,
entry_date date NOT NULL,
entry_time time without time zone NOT NULL,
event_time time without time zone NOT NULL,
day_volume bigint NOT NULL DEFAULT 0,
day_trade_ct integer,
entry_id bigint NOT NULL,
CONSTRAINT rt_h_nbbo_pkey PRIMARY KEY (entry_id),
CONSTRAINT rt_h_nbbo_pfkey FOREIGN KEY (product_id)
REFERENCES product (product_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
OIDS=FALSE
);
ALTER TABLE rt_h_nbbo
OWNER TO postgres;

_Index_:

CREATE INDEX rt_h_nbbo_idx
ON rt_h_nbbo
USING btree
(product_id, entry_date DESC);

_Test_:

SET track_io_timing = on;
EXPLAIN (ANALYZE,BUFFERS,VERBOSE,COSTS,TIMING) select * from rt_h_nbbo
where product_id=6508 and entry_date='2013-11-26';

_Output_:

"Index Scan using rt_h_nbbo_idx on public.rt_h_nbbo (cost=0.00..12768.21
rows=3165 width=76) (actual time=12.549..21654.547 rows=1631 loops=1)"
" Output: product_id, bid_price, bid_size, ask_price, ask_size,
last_price, entry_date, entry_time, event_time, day_volume,
day_trade_ct, entry_id"
" Index Cond: ((rt_h_nbbo.product_id = 6508) AND (rt_h_nbbo.entry_date
= '2013-11-26'::date))"
" Buffers: shared hit=4 read=1634"
" I/O Timings: read=21645.468"
"Total runtime: 21655.002 ms"

_Hardware_

Top of the line HP DL380 G7 server with 32 Gb Ram, P410i RAID, 10K SAS
drives in Raid-1 config. Wal on separate Raid-1 volume with 15K SAS drives.
The only unusual thing here is that I'm running on Windows Server 2008 R2.