Slow insert

Why is this so slow? Slow meaning 10 hours. Ans the further it gets, the slower it gets.

insert into TABLEA
select * from TABLEB
where KEY not in (select KEY from TABLEA )

Both tables are indexed on KEY. TABLEA has 9000 rows while TABLE B has 12000 rows. Received on Thu Dec 10 1998 - 19:05:19 CST