Query not using index

Here's the query:

SELECT
cart_row_id
FROM
pa_shopping_cart
WHERE
order_id = 20;

Here's the schema:

Attribute | Type |
Modifier
-----------------------+--------------------------+---------------------------------------------------------------------
cart_row_id | integer | not null default
product_id | integer | not null
color | character varying(100) |
size | character varying(100) |
style | character varying(100) |
order_id | integer | not null
time_added | timestamp with time zone |
voided_date | timestamp with time zone |
voided_by | integer |
expired_date | timestamp with time zone |
item_state | character varying(50) | default
'in_basket'
received_back_date | timestamp with time zone |
price_charged | numeric(30,6) |
price_refunded | numeric(30,6) |
shipping_charged | numeric(30,6) |
shipping_refunded | numeric(30,6) |
price_tax_charged | numeric(30,6) |
price_tax_refunded | numeric(30,6) |
shipping_tax_charged | numeric(30,6) |
shipping_tax_refunded | numeric(30,6) |
price_name | character varying(30) |
refund_id | integer |
cs_comments | text |
price | numeric(30,6) |
ship_group_id | integer |
ship_package_id | integer |
delivery_date | date |
sentiment | text |
vendor_id | integer |
linkshare_sent | bit(1) |
mapped_to_address | character(1) |
product_name | character varying(200) |
Indices: delivery_date_pa_shopping_cart_,
pa_cart_by_item_state,
pa_cart_by_order,
pa_cart_by_product,
pa_cart_row_order,
pa_item_map_to_addr,
pa_shop_cart_prod_ord_idx,
pa_shopping_cart_pkey,
ship_package_id_pa_shopping_car,
vendor_id_pa_shopping_cart_key

There is an index on:
just order_id
just order_id and cart_row_id
and a PK on cart row_id

I don't understand why it's not using one of these indexes!
Please post your responses to the group - my email is down.

Thanks for any help!

-r