Commit every N rows in PL/pgsql

I need to populate a new column in a Postgres 8.3 table. The SQL would be something like "update t set col_c = col_a + col_b". Unfortunately, this table has 110 million rows, so running that query runs out of memory.

In Oracle, I'd turn auto-commit off and write a pl/sql procedure that keeps a counter and commits every 10000 rows (pseudocode):

define cursor curs as select col_a from t

while fetch_from_cursor(curs) into a

     update t set col_c = col_a + col_b where col_a = a

     i++

     if i > 10000

         commit; i=0;

     end if;

     commit;

PL/pgsql doesn't allow that because it doesn't support nested transactions. 
Is there an equivalent Postgres way of doing this?

cheers,

Len
--
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