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https://issues.apache.org/jira/browse/FLINK-5564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15952127#comment-15952127
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Shaoxuan Wang commented on FLINK-5564:
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[~fhueske], implementing the entire codeGen into one PR results in thousands
lines of code changes. it is hard to be reviewed. I split FLINK-5813 into three
sub-tasks.
> User Defined Aggregates
> -----------------------
>
> Key: FLINK-5564
> URL: https://issues.apache.org/jira/browse/FLINK-5564
> Project: Flink
> Issue Type: Improvement
> Components: Table API & SQL
> Reporter: Shaoxuan Wang
> Assignee: Shaoxuan Wang
>
> User-defined aggregates would be a great addition to the Table API / SQL.
> The current aggregate interface is not well suited for the external users.
> This issue proposes to redesign the aggregate such that we can expose an
> better external UDAGG interface to the users. The detailed design proposal
> can be found here:
> https://docs.google.com/document/d/19JXK8jLIi8IqV9yf7hOs_Oz67yXOypY7Uh5gIOK2r-U/edit
> Motivation:
> 1. The current aggregate interface is not very concise to the users. One
> needs to know the design details of the intermediate Row buffer before
> implements an Aggregate. Seven functions are needed even for a simple Count
> aggregate.
> 2. Another limitation of current aggregate function is that it can only be
> applied on one single column. There are many scenarios which require the
> aggregate function taking multiple columns as the inputs.
> 3. “Retraction” is not considered and covered in the current Aggregate.
> 4. It might be very good to have a local/global aggregate query plan
> optimization, which is very promising to optimize UDAGG performance in some
> scenarios.
> Proposed Changes:
> 1. Implement an aggregate dataStream API (Done by
> [FLINK-5582|https://issues.apache.org/jira/browse/FLINK-5582])
> 2. Update all the existing aggregates to use the new aggregate dataStream API
> 3. Provide a better User-Defined Aggregate interface
> 4. Add retraction support
> 5. Add local/global aggregate
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