+--------------+--------------------------+--------------+--------------+
| request_id   | impressions              | event_item   | event_type   |
|--------------+--------------------------+--------------+--------------|
| r1           | ['i1', 'i2', 'i3', 'i4'] |              | impress      |
| r2           | ['i2', 'i3', 'i4', 'i1'] | i2           | click        |
| r3           | ['i3', 'i4', 'i1', 'i2'] |              | impress      |
| r4           | ['i4', 'i1', 'i2', 'i3'] | i3           | click        |
| r5           | ['i1', 'i2', 'i3', 'i4'] |              | impress      |
+--------------+--------------------------+--------------+--------------+