+--------------+--------+------------+--------------+
| request_id   | item   |   position | event_type   |
|--------------+--------+------------+--------------|
| r2           | i2     |          1 | impress      |
| r2           | i3     |          2 | impress      |
| r2           | i4     |          3 | impress      |
| r2           | i1     |          4 | impress      |
| r2           | i2     |          1 | click        |
+--------------+--------+------------+--------------+