[jira] [Commented] (FLINK-8903) Built-in agg functions VAR_POP, VAR_SAMP, STDEV_POP, STDEV_SAMP are broken in Group Windows

["Shuyi Chen (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-8903?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16400925#comment-16400925 ] 

Shuyi Chen commented on FLINK-8903:
-----------------------------------

[~fhueske], I can help take a look at this issue.

> Built-in agg functions VAR_POP, VAR_SAMP, STDEV_POP, STDEV_SAMP are broken in Group Windows
> -------------------------------------------------------------------------------------------
>
>                 Key: FLINK-8903
>                 URL: https://issues.apache.org/jira/browse/FLINK-8903
>             Project: Flink
>          Issue Type: Bug
>          Components: Table API & SQL
>    Affects Versions: 1.3.2, 1.5.0, 1.4.2
>            Reporter: lilizhao
>            Assignee: Shuyi Chen
>            Priority: Critical
>             Fix For: 1.5.0
>
>         Attachments: QQ图片20180312180143.jpg, TableAndSQLTest.java
>
>
> The built-in aggregation functions VAR_POP, VAR_SAMP, STDEV_POP, STDEV_SAMP are translated into regular AVG functions if they are applied in the context of a Group Window aggregation (\{{GROUP BY TUMBLE/HOP/SESSION}}).
> The reason is that these functions are internally represented as {{SqlAvgAggFunction}} but with different {{SqlKind}}. When translating Calcite aggregation functions to Flink Table agg functions, we only look at the type of the class, not at the value of the {{kind}} field. We did not notice that before, because in all other cases (regular {{GROUP BY}} without windows or {{OVER}} windows, we have a translation rule {{AggregateReduceFunctionsRule}} that decomposes the more complex functions into expressions of {{COUNT}} and {{SUM}} functions such that we never execute an {{AVG}} Flink function. That rule can only be applied on {{LogicalAggregate}}, however, we represent group windows as {{LogicalWindowAggregate}}, so the rule does not match.
> We should fix this by:
> 1. restrict the translation to Flink avg functions in {{AggregateUtil}} to {{SqlKind.AVG}}. 
> 2. implement a rule (hopefully based on {{AggregateReduceFunctionsRule}}) that decomposes the complex agg functions into the {{SUM}} and {{COUNT}}.
> Step 1. is easy and a quick fix but we would get an exception "Unsupported Function" if {{VAR_POP}} is used in a {{GROUP BY}} window.
> Step 2. might be more involved, depending on how difficult it is to port the rule.
>  



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