create sequence g; commit;
recreate table t( id int primary key, s varchar(36) ); commit;
insert into t
select i,s from
(
  select gen_id(g,1) i, uuid_to_char(gen_uuid()) s
  from rdb$types a,rdb$types b,(select 1 i from rdb$types rows 20) rows 10000
)
order by rand();
commit;

select count(*) from (
  select x.id,z.s
  from (select id, rdb$db_key k from t a order by id) x
  -- left
  join t z on x.k = z.rdb$db_key
);

Statement 150:
---------------------------------------------------------------
select count(*) from (
  select x.id,z.s
  from (select id, rdb$db_key k from t a order by id) x
  join t z on x.k = z.rdb$db_key
)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PLAN JOIN (X A ORDER RDB$PRIMARY9, Z NATURAL)
1 records fetched
   49146  ms, 204300015 fetch(es)

Table                             Natural     Index    Update  
***************************************************************
T                               100000000     10000

Statement 26:
---------------------------------------------------------
select count(*) from (
  select x.id,z.s
  from (select id, rdb$db_key k from t a order by id) x
  join t z on x.k = z.rdb$db_key
)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Select Expression
    -> Aggregate
        -> Filter
            ->  Nested Loop Join (inner)
                -> Table "T" as "X A" Access By ID
                    -> Index "RDB$PRIMARY2" Full Scan
                -> Table "T" as "Z" Full Scan
1 records fetched
   67697  ms, 204300015 fetch(es)

Table                             Natural     Index    Up
*********************************************************
T                               100000000     10000

select count(*) from (
  select x.id,(select z.s from t z where z.rdb$db_key = x.k) s
  from (select id, rdb$db_key k from t a order by id) x
  --left
  --join t z on x.k = z.rdb$db_key
)

Select Expression
    -> Singularity Check
        -> Filter
            -> Table "T" as "Z" Access By ID
                -> DBKEY
Select Expression
    -> Aggregate
        -> Materialize
            -> Table "T" as "X A" Access By ID
                -> Index "RDB$PRIMARY2" Full Scan
1 records fetched
     31 ms, 50015 fetch(es)

Table                             Natural     Index 
****************************************************
T                                             20000