Updating a specific number of rows in pl/pgsql

I'm working on a queuing application. As part of this I'm trying to
write a pl/pgsql function that updates a specific number of rows in the
most efficient way possible. Multiple queues are contained within a
single table.

I can get the rows I want to update like this:

SELECT *
FROM queue
WHERE id = p_queue_id
ORDER BY rank
LIMIT p_number_of_items;

Of course, there may not be p_number_of_items available in the queue.

I want to update all the rows in the cursor in the same way:

UPDATE queue SET assigned = TRUE;

The "obvious" solution is to get a cursor on the query and attempt to
MOVE through that cursor in a loop, using the row count from the SELECT
to tell me when I am done. I can then use UPDATE ... WHERE CURRENT OF
... to do the updates. This seems cumbersome and inefficient to me. Is
there a better way?

Ideally, I'd like to do something like:

UPDATE (SELECT ... ) ...

--
Peter Headland
Architect
Actuate Corporation