> However in FB.2.0.0.0 the same delete SQL
>
> DELETE FROM TICKETITEMS WHERE TICKET_UNIQUENUM
> NOT IN (SELECT TICKET_UNIQUENUM FROM TICKETS);
>
> execution takes forever, since it selects
>
> PLAN (TICKETS NATURAL)
> PLAN (TICKETITEMS NATURAL)
>

I am not sure, but try rewrite delete statement :

DELETE FROM TICKETITEMS WHERE not exixts(
select * from TICKETS where
TICKET_UNIQUENUM=TICKETITEMS.TICKET_UNIQUENUM );

-Laco.