Add finalize() interface to UDF
Issue Type: New Feature
Reporter: Yiping Han
I would like to have a finalize() method to UDF, which will be called when no
more inputs and the UDF will be killed. The finalize() method should allow to
generate extra output, which in many cases could benefit aggregations.
There are couple of application that can benefit from this feature.
One of the example is, in some UDFs, I need to open some resource(i. e. local
file) and when the task finishes, I need to close the resource.
Another example is, in one of my application, I do statistics for a list of
categories and I need to generate a summary category and attach to the end of
the table. With the finalize method, I could achieve this in an efficient and
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