Found one difference between F2.0 optimizer and FB1.5

select tt.* from ticketitems tt where tt.ticket_uniquenum
not in (select t.ticket_uniquenum from tickets t )
PLAN (T NATURAL)
PLAN (TT NATURAL)

select tt.* from ticketitems tt where tt.ticket_uniquenum
in (select t.ticket_uniquenum from tickets t )
PLAN (T INDEX (RDB$PRIMARY15))
PLAN (TT NATURAL)

select tt.* from ticketitems tt where tt.ticket_uniquenum
not in (select t.ticket_uniquenum from tickets t
where t.ticket_uniquenum = tt.ticket_uniquenum)
PLAN (T INDEX (RDB$PRIMARY15))
PLAN (TT NATURAL)

select tt.* from ticketitems tt where tt.ticket_uniquenum
in (select t.ticket_uniquenum from tickets t
where t.ticket_uniquenum = tt.ticket_uniquenum)
PLAN (T INDEX (RDB$PRIMARY15))
PLAN (TT NATURAL)

FB1.5 uses and indexed plan for all of them.