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https://issues.apache.org/jira/browse/FLINK-4691?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15568264#comment-15568264
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ASF GitHub Bot commented on FLINK-4691:
---------------------------------------

Github user fhueske commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2562#discussion_r82971688
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/api/table/runtime/aggregate/PropertyRead.scala
 ---
    @@ -0,0 +1,33 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + *     http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.flink.api.table.runtime.aggregate
    +
    +import org.apache.flink.streaming.api.windowing.windows.Window
    +
    +/**
    +  * Base class for reading a window property. The property will be 
extracted once and
    +  * can be read multiple times.
    +  */
    +trait PropertyRead[T] extends Serializable {
    +
    +  def extract(window: Window): Unit
    --- End diff --
    
    I think we do not need a generic interface to extract properties. The 
`extract` method requires a window object but not all properties might be 
present in the window object. So I am not sure how much generalization is 
required here.
    
    I think we can implement a `TimeWindowPropertyCollector` that works as 
follows:
    ```
    class TimeWindowPropertyCollector(start: Int, end: Int) extends 
Collector[Row] {
      var finalCollector: Collector[Row] = _
      var timeWindow: TimeWindow = _
    
      override def collect(record: Row): Unit = {
    
        if (start > -1) {
          record.setField(start, timeWindow.getStart)
        }
        if (end > -1) {
          record.setField(end, timeWindow.getEnd)
        }
    
        finalCollector.collect(record)
      }
    
      override def close(): Unit = finalCollector.close()
    }
    ```
    
    If we (ever) need more properties, we can add another wrapping collector or 
change the design. But at this point I think this is sufficient. 
    
    What do you think?


> Add group-windows for streaming tables        
> ---------------------------------------
>
>                 Key: FLINK-4691
>                 URL: https://issues.apache.org/jira/browse/FLINK-4691
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Table API & SQL
>            Reporter: Timo Walther
>            Assignee: Timo Walther
>
> Add Tumble, Slide, Session group-windows for streaming tables as described in 
> [FLIP-11|https://cwiki.apache.org/confluence/display/FLINK/FLIP-11%3A+Table+API+Stream+Aggregations].
>  
> Implementation of group-windows on streaming tables. This includes 
> implementing the API of group-windows, the logical validation for 
> group-windows, and the definition of the “rowtime” and “systemtime” keywords. 
> Group-windows on batch tables won’t be initially supported and will throw an 
> exception.



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