looking at the docs for org.apache.spark.sql.expressions.Aggregator it says for reduce method: "For performance, the function may modify `b` and return it instead of constructing new object for b.".
it makes no such comment for the merge method. this is surprising to me because i know that for PairRDDFunctions.aggregateByKey mutation is allowed in both seqOp and combOp (which are the equivalents of reduce and merge in Aggregator). is it safe to mutate b1 and return it in Aggregator.merge?
