Oracle generators

gen.sql

set echo on timing on;

with 
r1 as (select level from dual connect by level <= 100),
r2 as (select level from dual connect by level <= 100),
r3 as (select level from dual connect by level <= 1000)
select sum(rownum) from r1, r2, r3;

select sum(i) from xmltable('1 to 10000000' columns i for ordinality);
set echo off timing off autot off

Result.sql

SQL> with
  2  r1 as (select level from dual connect by level <= 100),
  3  r2 as (select level from dual connect by level <= 100),
  4  r3 as (select level from dual connect by level <= 1000)
  5  select sum(rownum) from r1, r2, r3;

SUM(ROWNUM)
-----------
 5.0000E+13

Elapsed: 00:00:03.32
SQL>
SQL> select sum(i) from xmltable('1 to 10000000' columns i for ordinality);

    SUM(I)
----------
5.0000E+13

Elapsed: 00:00:11.77