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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