slow simple update?

Hi there,

I'm doing an update of ~30,000 rows and she takes about 15mins on
pretty good hardware, even just after a vacuum analyze.
I was hoping some kind soul could offer some performance advice. Do I
just have too many indexes? Or am I missing some trick with the nulls?


MY QUERY
========
update bob.product_price set thru_date = '2005-06-22 22:08:49.957'
where thru_date is null;


MY TABLE
=========
                   Table "bob.product_price"
           Column            |           Type           | Modifiers
-----------------------------+--------------------------+-----------
 product_id                  | character varying(20)    | not null
 product_price_type_id       | character varying(20)    | not null
 currency_uom_id             | character varying(20)    | not null
 product_store_id            | character varying(20)    | not null
 from_date                   | timestamp with time zone | not null
 thru_date                   | timestamp with time zone |
 price                       | numeric(18,2)            |
 created_date                | timestamp with time zone |
 created_by_user_login       | character varying(255)   |
 last_modified_date          | timestamp with time zone |
 last_modified_by_user_login | character varying(255)   |
 last_updated_stamp          | timestamp with time zone |
 last_updated_tx_stamp       | timestamp with time zone |
 created_stamp               | timestamp with time zone |
 created_tx_stamp            | timestamp with time zone |

Indexes:
---------
pk_product_price primary key btree
 (product_id, product_price_type_id, currency_uom_id,
product_store_id, from_date),
prdct_prc_txcrts btree (created_tx_stamp),
prdct_prc_txstmp btree (last_updated_tx_stamp),
prod_price_cbul btree (created_by_user_login),
prod_price_cuom btree (currency_uom_id),
prod_price_lmbul btree (last_modified_by_user_login),
prod_price_prod btree (product_id),
prod_price_pst btree (product_store_id),
prod_price_type btree (product_price_type_id)

Foreign Key constraints:
-------------------------
prod_price_prod FOREIGN KEY (product_id) REFERENCES bob.product(product_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION,
prod_price_type FOREIGN KEY (product_price_type_id) REFERENCES
bob.product_price_type(product_price_type_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION,
prod_price_cuom FOREIGN KEY (currency_uom_id) REFERENCES bob.uom(uom_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION,
prod_price_pst FOREIGN KEY (product_store_id) REFERENCES
bob.product_store(product_store_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION,
prod_price_cbul FOREIGN KEY (created_by_user_login) REFERENCES
bob.user_login(user_login_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION,
prod_price_lmbul FOREIGN KEY (last_modified_by_user_login) REFERENCES
bob.user_login(user_login_id)
 ON UPDATE NO ACTION ON DELETE NO ACTION