Timo Walther commented on FLINK-4557:

[~shijinkui] I will create issues for the subtasks once the FLIP-11 is not in 
"discuss" state anymore.

> Table API Stream Aggregations
> -----------------------------
>                 Key: FLINK-4557
>                 URL: https://issues.apache.org/jira/browse/FLINK-4557
>             Project: Flink
>          Issue Type: New Feature
>          Components: Table API & SQL
>            Reporter: Timo Walther
> The Table API is a declarative API to define queries on static and streaming 
> tables. So far, only projection, selection, and union are supported 
> operations on streaming tables.
> This issue and the corresponding FLIP proposes to add support for different 
> types of aggregations on top of streaming tables. In particular, we seek to 
> support:
> *Group-window aggregates*, i.e., aggregates which are computed for a group of 
> elements. A (time or row-count) window is required to bound the infinite 
> input stream into a finite group.
> *Row-window aggregates*, i.e., aggregates which are computed for each row, 
> based on a window (range) of preceding and succeeding rows.
> Each type of aggregate shall be supported on keyed/grouped or 
> non-keyed/grouped data streams for streaming tables as well as batch tables.
> Since time-windowed aggregates will be the first operation that require the 
> definition of time, we also need to discuss how the Table API handles time 
> characteristics, timestamps, and watermarks.
> The FLIP can be found here: 
> https://cwiki.apache.org/confluence/display/FLINK/FLIP-11%3A+Table+API+Stream+Aggregations

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