Referencing tables are grinding UPDATE to a halt

Hey all!

I'm having some trouble with a simple update on a table that only has 
about 250,000 rows in it.  The table itself looks something like:

CREATE TABLE price_details (
price_detail_id int PRIMARY KEY,
private bool
) ;

There is one table that references price_details, but isn't affected 
by the "private" column, and one table that references this second 
table.  They look something like:

CREATE TABLE prices (
p_id int PRIMARY KEY,
price_detail_id int NOT NULL REFERENCES price_details ON DELETE CASCADE,
max_sales int
) ;

CREATE INDEX prices_price_detail_id  ON prices (price_detail_id) ;

CREATE TABLE sales (
sales_id int PRIMARY KEY,
p_id int NOT NULL REFERENCES prices ON DELETE CASCADE,
sales int
) ;

CREATE INDEX sales_p_id ON sales (p_id) ;

I'm trying to do a simple update to the "private" column in the 
price_details table, which I expected to take a few seconds at 
most.  After 10 minutes, I gave up and ran explain, with this as the result:


EXPLAIN UPDATE price_details SET private = 't' WHERE private = 'f' ;
---------------------------------------------------------------------------------------- Nested Loop
(cost=2663.45..363527947.70rows=118759098 width=50)   Join Filter: (subplan)   ->  Seq Scan on sales
(cost=0.00..3685.27rows=54627 width=42)   ->  Materialize  (cost=2663.45..2706.93 rows=4348 width=12)         ->  Seq
Scanon price_details  (cost=0.00..2663.45 
 
rows=4348 width=12)               Filter: (private  = false)   SubPlan     ->  Index Scan using prices_price_detail_id
on
 
prices  (cost=0.00..3.01 rows=1 width=4)           Index Cond: (price_detail_id = $0)
 Seq Scan on price_details  (cost=0.00..2663.45 rows=4348 width=508)   Filter: (private = false)
(12 rows)
----------------------------------------------------------------------------------------

So it looks to me like the postgres is checking this table against 
the table that references it, and the table that reference that 
table, making what should be a very easy transaction into something 
unusable.  Is there any way to avoid this without losing proper referencing?

Any suggestions would be appreciated.

-Scott