Monitoring buffercache...

Hi All;

I've installed pg_buffercache and I want to use it to help define the optimal
shared_buffers size.

Currently I run this each 15min via cron:
insert into buffercache_stats select now(), isdirty, count(*) as buffers,
(count(*) * 8192) as memory from pg_buffercache group by 1,2;

and here's it's explain plan
explain insert into buffercache_stats select now(), isdirty, count(*) as
buffers, (count(*) * 8192) as memory from pg_buffercache group by 1,2;
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
 Subquery Scan "*SELECT*"  (cost=65.00..65.23 rows=2 width=25)
   ->  HashAggregate  (cost=65.00..65.12 rows=2 width=1)
         ->  Function Scan on pg_buffercache_pages p  (cost=0.00..55.00
rows=1000 width=1)
(3 rows)


Then once a day I will pull a report from the buffercache_stats table. The
buffercache_stats table is our own creation :

\d buffercache_stats
             Table "public.buffercache_stats"
     Column     |            Type             | Modifiers
----------------+-----------------------------+-----------
 snap_timestamp | timestamp without time zone |
 isdirty                    | boolean                     |
 buffers                  | integer                       |
 memory                | integer                       |


Here's my issue, the server that we'll eventually roll this out to is
extremely busy and the every 15min query above has the potential to have a
huge impact on performance.

Does anyone have any suggestions per a better approach or maybe a way to
improve the performance for the above query ?

Thanks in advance...