private static Tensor generalJoin(Tensor a, Tensor b, TensorType joinedType, DoubleBinaryOperator combinator) {
        if (a instanceof IndexedTensor && b instanceof IndexedTensor)
            return indexedGeneralJoin((IndexedTensor) a, (IndexedTensor) b, joinedType, combinator);
        else
            return mappedHashJoin(a, b, joinedType, combinator);
    }
Slow join which works for any two tensors
vespajlib/src/main/java/com/yahoo/tensor/functions/Join.java