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It doesn't look like it is documented to me.

I'm not sure about this particular case, although, usually if something like this is undocumented it's because the API isn't stable enough yet. So it might be that. I think @Clint Wylie wrote it so he might have more info

yeah, I think it has stabilized now probably, didn't document at first for that reason

Thanks very much, really appreciate the replies!

https://github.com/apache/druid/blob/master/processing/src/main/java/org/apache/druid/query/aggregation/ExpressionLambdaAggregatorFactory.java#L103 for the current set of properties it accepts