UPDATE with table join

I need to update some records in a table depending on values in another
table:

UPDATE
item SET export_time = now()
WHERE item_pk IN
(
SELECT
item.item_pk AS pk
FROM ITEM
LEFT JOIN product product ON Item.product_fk = product.product_pk
WHERE product.xyz = 't' AND ...
)

Is this the most efficient way of doing this?
Is there a limit to the number of results that IN can cope with?

This needs to run on Postgres 7.4.

--

Best Regards,

Tarlika Elisabeth Schmitz

A: Because it breaks the logical sequence of discussion
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