Window Top-N

Function

Window Top-N is a special Top-N which returns the N smallest or largest values for each window and other partitioned keys.

Unlike regular Top-N on continuous tables, window Top-N does not emit intermediate results but only a final result, the total top N records at the end of the window. Moreover, window Top-N purges all intermediate state when no longer needed.

Window Top-N queries have better performance if users do not need results updated per record. Usually, Window Top-N is used with Windowing Table-Valued Functions (Windowing TVFs) directly. Besides, Window Top-N could be used with other operations based on Windowing Table-Valued Functions (Windowing TVFs), such as Window Aggregation, Window TopN and Window Join.

Window Top-N can be defined in the same syntax as regular Top-N, see Top-N documentation for more information. Besides that, Window Top-N requires the PARTITION BY clause contains window_start and window_end columns of the relation applied Windowing TVF or Window Aggregation. Otherwise, the optimizer will not be able to translate the query.

For more information, see Window Top-N.

Syntax

SELECT [column_list]
FROM (
   SELECT [column_list],
     ROW_NUMBER() OVER (PARTITION BY window_start, window_end [, col_key1...]
       ORDER BY col1 [asc|desc][, col2 [asc|desc]...]) AS rownum
   FROM table_name) -- relation applied windowing TVF
WHERE rownum <= N [AND conditions]

Caveats

Flink only supports Window Top-N follows after Windowing TVF with Tumble Windows, Hop Windows and Cumulate Windows.

Example

Window Top-N follows after Window Aggregation

The following example shows how to calculate Top 3 suppliers who have the highest sales for every tumbling 10 minutes window.

-- tables must have time attribute, e.g. `bidtime` in this table
Flink SQL> desc Bid;
+-------------+------------------------+------+-----+--------+---------------------------------+
|        name |                   type | null | key | extras |                       watermark |
+-------------+------------------------+------+-----+--------+---------------------------------+
|     bidtime | TIMESTAMP(3) *ROWTIME* | true |     |        | `bidtime` - INTERVAL '1' SECOND |
|       price |         DECIMAL(10, 2) | true |     |        |                                 |
|        item |                 STRING | true |     |        |                                 |
| supplier_id |                 STRING | true |     |        |                                 |
+-------------+------------------------+------+-----+--------+---------------------------------+

Flink SQL> SELECT * FROM Bid;
+------------------+-------+------+-------------+
|          bidtime | price | item | supplier_id |
+------------------+-------+------+-------------+
| 2020-04-15 08:05 |  4.00 |    A |   supplier1 |
| 2020-04-15 08:06 |  4.00 |    C |   supplier2 |
| 2020-04-15 08:07 |  2.00 |    G |   supplier1 |
| 2020-04-15 08:08 |  2.00 |    B |   supplier3 |
| 2020-04-15 08:09 |  5.00 |    D |   supplier4 |
| 2020-04-15 08:11 |  2.00 |    B |   supplier3 |
| 2020-04-15 08:13 |  1.00 |    E |   supplier1 |
| 2020-04-15 08:15 |  3.00 |    H |   supplier2 |
| 2020-04-15 08:17 |  6.00 |    F |   supplier5 |
+------------------+-------+------+-------------+

Flink SQL> SELECT *
  FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY window_start, window_end ORDER BY price DESC) as rownum
    FROM (
      SELECT window_start, window_end, supplier_id, SUM(price) as price, COUNT(*) as cnt
      FROM TABLE(
        TUMBLE(TABLE Bid, DESCRIPTOR(bidtime), INTERVAL '10' MINUTES))
      GROUP BY window_start, window_end, supplier_id
    )
  ) WHERE rownum <= 3;
+------------------+------------------+-------------+-------+-----+--------+
|     window_start |       window_end | supplier_id | price | cnt | rownum |
+------------------+------------------+-------------+-------+-----+--------+
| 2020-04-15 08:00 | 2020-04-15 08:10 |   supplier1 |  6.00 |   2 |      1 |
| 2020-04-15 08:00 | 2020-04-15 08:10 |   supplier4 |  5.00 |   1 |      2 |
| 2020-04-15 08:00 | 2020-04-15 08:10 |   supplier2 |  4.00 |   1 |      3 |
| 2020-04-15 08:10 | 2020-04-15 08:20 |   supplier5 |  6.00 |   1 |      1 |
| 2020-04-15 08:10 | 2020-04-15 08:20 |   supplier2 |  3.00 |   1 |      2 |
| 2020-04-15 08:10 | 2020-04-15 08:20 |   supplier3 |  2.00 |   1 |      3 |
+------------------+------------------+-------------+-------+-----+--------+