Test case 1 (with union):

select ( select first 1 r.rdb$relation_name
         from rdb$relations r
         where r.rdb$relation_id = d.rdb$relation_id - 1 )
from (
  select * from rdb$database
  union all
  select * from rdb$database
) d

PLAN (R NATURAL)
-- R is scanned naturally
PLAN (D RDB$DATABASE NATURAL)
PLAN (D RDB$DATABASE NATURAL)


But (without union):

select ( select first 1 r.rdb$relation_name
         from rdb$relations r
         where r.rdb$relation_id = d.rdb$relation_id - 1 )
from (
  select * from rdb$database
) d

PLAN (R INDEX (RDB$INDEX_1))
-- R is read through the index
PLAN (D RDB$DATABASE NATURAL)

Test case 2 (with union):

select 1
from (
  select * from rdb$relations
  union all
  select * from rdb$relations
) r
where exists ( select * from rdb$relation_fields f where f.rdb$relation_name = r.rdb$relation_name )

PLAN (F NATURAL)
PLAN (R RDB$RELATIONS NATURAL)
PLAN (R RDB$RELATIONS NATURAL)

But (without union):

select 1
from ( select * from rdb$relations ) r
where exists ( select * from rdb$relation_fields f where f.rdb$relation_name = r.rdb$relation_name )

PLAN (F INDEX (RDB$INDEX_4))
PLAN (R RDB$RELATIONS NATURAL)