erikerlandson edited a comment on issue #25024: [SPARK-27296][SQL] User Defined Aggregators that do not ser/de on each input row URL: https://github.com/apache/spark/pull/25024#issuecomment-544016559 > I wonder why those encoders aren't defaulted in `Aggregator` In theory it would be to allow custom encoders, however if you look at the very sparse signature of `Encoder` trait, combined with how the spark sql code assumes variations on `ExpressionEncoder`, I don't think that is feasible. I agree that the `Aggregator` and `Encoder` APIs should be revisited, although that's a topic for some other PR. This is wandering away from the PR at hand, but the contravariant `IN` type parameter prevents any method in `Aggreator` from binding a `TypeTag`, and otherwise the `agg($"x")` method could be supported right on `Aggregator`, but currently that isn't possible.
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