SQL> recreate table tmp(id int, x int); commit;
SQL> insert into tmp select row_number()over(),rand()*100 from rdb$types,rdb$types rows 10000; commit;
SQL> set list on;
SQL> select count(*) from tmp;

COUNT                            10000 

SQL> set planonly;
SQL> set explain on;
SQL> select d.id, b.rx
CON> from ( select id, row_number()over() rn from tmp ) d
CON> join ( select cast(round((0.5+rand()* 10000  ),0) as int) rx from rdb$database ) b
CON> on d.rn = b.rx
CON> ;

Select Expression
    -> Filter
        ->  Nested Loop Join (inner)
            ->  Window 
                -> Record Buffer (record length: 25)
                    -> Table "D TMP" Full Scan
            -> Table "B RDB$DATABASE" Full Scan

SQL> select d.id, b.rx
CON> from ( select id,s um(id)over() s  from tmp ) d
CON> join ( select cast(round((0.5+rand()*10000 ),0) as int) rx from rdb$database ) b
CON> on d.id = b.rx;

Select Expression
    -> Filter
        ->  Nested Loop Join (inner)
            ->  Window 
                -> Record Buffer (record length: 25)
                    -> Table "D TMP" Full Scan
            -> Table "B RDB$DATABASE" Full Scan

SQL> select d.id, b.rx
CON> from ( select id from tmp ) d
CON> join ( select cast(round((0.5+rand()*10000 ),0) as int) rx from rdb$database ) b
CON> on d.id = b.rx;

Select Expression
    -> Filter
        ->  Nested Loop Join (inner)
            ->  Table "B RDB$DATABASE" Full Scan 
            -> Table "D TMP" Full Scan