[GitHub] flink issue #3175: [FLINK-5584]support sliding-count row-window on streaming...

[wuchong <gi...@git.apache.org>]

Github user wuchong commented on the issue:

    https://github.com/apache/flink/pull/3175
  
    Hi @hongyuhong , thank your for your job. But it seems that you misunderstand the SQL OVER syntax. 
    The OVER clause defines a window or user-specified set of rows within a query result set. A window function then computes a value for each row in the window. It is similar to Row-Window proposed in [FLIP-11](https://cwiki.apache.org/confluence/display/FLINK/FLIP-11%3A+Table+API+Stream+Aggregations), but is different with Sliding Row-count window.
    
    For example, OVER (ROWS 2 PRECEDING) means that the window of rows that the function operates on is three rows in size, starting with 2 rows preceding until and including the current row.
    
    Say we have a table `T1` 
    
    ```
    t  a  
    -----
    1  1 
    2  5 
    3  3 
    4  5 
    5  4 
    6 11
    ```
    
    and the following SQL will yield:
    
    ```sql
    SELECT t, a, sum(a) OVER (ROWS 2 PRECEDING) FROM T1
    ```
    
    ```
    t  a  avg
    ----------
    1  1  1
    2  5  6
    3  3  9
    4  5  13
    5  4  12
    6 11  20
    ```
    
    For Row-window, we would need something more complex, especially when we need to order by timestamp. For example, to support event-time count-window row-window, we need to create a custom operator that collects records in a priority queue ordered by timestamp. Once a watermark is received for the upper bound of a window, the priority queue is used to evaluate the window function (based on count) and to purge too old records. 
    
    I would suggest this PR to wait for FLINK-4679. When FLINK-4679 is fixed, this PR can be easily supported IMO.


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