godfreyhe commented on issue #9141: [FLINK-12249][table] Fix type equivalence 
check problems for Window Aggregates
URL: https://github.com/apache/flink/pull/9141#issuecomment-513070691
 
 
   > @dawidwys Thanks for your comments. I think you are right.
   > 
   > We had a discussion just now. (with @godfreyhe @wuchong @sunjincheng121 ) 
We think that, to solve the problem fundamentally, the current window aggregate 
should not extend from `org.apache.calcite.rel.core.Aggregate`, instead it 
should extend from `SingleRel`. This makes sure the right semantics of window 
aggregate.
   > 
   > To correct this, the changes may be somehow very large as there are many 
existing window related rules. Also, this is a problem a long time ago and the 
probability of occurrence is relatively small.
   > 
   > Considering the reasons above, I think it would be good if we can fix this 
issue later after release-1.9?
   > 
   > What do you think? @dawidwys @godfreyhe @wuchong @sunjincheng121
   > 
   > Best, Hequn
   
   yes, it's a big issue to refactor WindowAggregate completely using solution3 
mentioned in [FLINK-12249](https://issues.apache.org/jira/browse/FLINK-12249). 
"Creating special window aggregate call function" is not a proper approach, how 
about users defined aggregate function? currently, i think we should make 
WindowAggregate does not extend from Aggregate in this pr, which had been 
validated in  a blink minor branch. and do the clean refactor after 1.9 ?

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