If you have a unique key, you may use COUNT(*) OVER ( ORDER BY unique_key ) as ROWNUM
SELECT t.*, count(*) OVER (ORDER BY k ) ROWNUM
FROM yourtable t;
| k | n | rownum |
|---|-------|--------|
| a | TEST1 | 1 |
| b | TEST2 | 2 |
| c | TEST2 | 3 |
| d | TEST4 | 4 |