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
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.
1. Implement an aggregate dataStream API (Done by 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