Github user yanboliang commented on the issue:

    https://github.com/apache/spark/pull/18798
  
    @WeichenXu123 @thunterdb 
    Thanks for this great work, we are always happy to see improvement which 
can help us to migrate MLlib workload to Dataset based API.
    Here are my two cents:
    1, Form the performance test result, there are performance degradation from 
3 -> 2 -> 1.
    The difference between case 3 and case 2 is the deserialization cost of 
each instance, I suspect we can't skip this step, so it maybe more hard to 
optimize it. However, the difference between case 2 and case 1 is the aggregate 
implementation. The aggregate on Dataset will use user-defined type as 
aggregate buffer, so I think we should make some effort to improve 
```TypedImperativeAggregate```. I guess it maybe not so hard compared with the 
previous bottleneck, but this is just my intuition, let’s check with SQL 
guys. cc @liancheng  @cloud-fan @hvanhovell 
    2, @WeichenXu123 In the current experiment, I saw you only use two 
partitions. Is there any difference if we run against more partitions or more 
dataset?
    3, @thunterdb I agree that we can get this in and make improvement 
continuously. When we get desirable result, we can start to migrate other MLlib 
workload on top of Dataset/catalysts. But we should add comment to let users 
know(if we can’t get desirable result before 
    2.3.0) that the performance of Dataset-based multivariate statistic is not 
good enough currently. Thanks.



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