Custom aggregate reducers receive every value in a bucket (including
undefined) and return a single result. The return type is widened to
ColumnValue so reducers may emit an array — the resulting column's
schema kind is inferred as 'array' when the custom reducer output is
declared via AggregateOutputSpec.kind.
Custom aggregate reducers receive every value in a bucket (including
undefined) and return a single result. The return type is widened toColumnValueso reducers may emit an array — the resulting column's schema kind is inferred as'array'when the custom reducer output is declared viaAggregateOutputSpec.kind.