Spark Dataset aggregation similar to RDD aggregate(zero)(accum, combiner)

问题

RDD has a very useful method aggregate that allows to accumulate with some zero value and combine that across partitions. Is there any way to do that with Dataset[T]. As far as I see the specification via Scala doc, there is actually nothing capable of doing that. Even the reduce method allows to do things only for binary operations with T as both arguments. Any reason why? And if there is anything capable of doing the same?

Thanks a lot!

VK

回答1:

There are two different classes which can be used to achieve aggregate-like behavior in Dataset API:

• UserDefinedAggregateFunction which uses SQL types and takes Columns as an input.

  Initial value is defined using initialize method, seqOp with update method and combOp with merge method.

  Example implementation: How to define a custom aggregation function to sum a column of Vectors?

• Aggregator which uses standard Scala types with Encoders and takes records as an input.

  Initial value is defined using zero method, seqOp with reduce method and combOp with merge method.

  Example implementation: How to find mean of grouped Vector columns in Spark SQL?

Both provide additional finalization method (evaluate and finish respectively) which is used to generate final results and can be used for both global and by-key aggregations.

来源：https://stackoverflow.com/questions/42378806/spark-dataset-aggregation-similar-to-rdd-aggregatezeroaccum-combiner