presto-ml/src/main/java/io/prestosql/plugin/ml/LearnClassifierAggregation.java/LearnClassifierAggregation/combine
class LearnClassifierAggregation:    @CombineFunction
    public static void combine(@AggregationState LearnState state, @AggregationState LearnState otherState)
    {
        throw new UnsupportedOperationException("LEARN must run on a single machine");
    }