have tried to make such query:

select * from TABLE1 a
left join ( select * from TABLE2 a2 where a2."Val" = :Val and a2."Pos"
= :Pos ) b
on a."ID" = b."ID_ref"

it seems, that FB1.5 don't support such subqueries (memory tables),
how to rewrite query to get working FB1.5 query?