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From "Fabian Hueske (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5564) User Defined Aggregates
Date Mon, 06 Feb 2017 17:19:41 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15854402#comment-15854402

Fabian Hueske commented on FLINK-5564:

Hi [~Shaoxuan Wang], I think it should be possible to split the first three steps as follows:

1. Add the new UDAGG interface and migrate existing aggregation functions to it.
We keep the current implementation and just add code and unit tests for the implementations
of the aggregation functions that implement the new interface.
2. Use the new aggregation function for batch tables.
We switch the implementation for the DataSet runtime code. We still keep the old aggregation
functions for the streaming code.
3. Use the new aggregation function for streaming tables.
We switch the implementation for the DataStream runtime code. In this step we remove the old
aggregation functions and clean up.

Adressing 1, 2, and 3 in a single issue will result in a huge PR which will be hard to review.
I'd prefer several smaller steps with well defined scope.

Regarding the discussion of the window OVER functions. It would be great if you could post
your comment (with which I agree) to the corresponding JIRA issue and the discussion on the
dev list otherwise it might not be noticed.

Thanks, Fabian

> 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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