Github user gatorsmile commented on the pull request:
https://github.com/apache/spark/pull/9419#issuecomment-153148409
@hvanhovell Your understanding is right.
If we merge both grouping and aggregation together, it will introduce extra
complexity to generate the logical plan for the following case:
"select a + b, b, sum(a - b), sum(a) from mytable group by a + b, b with
rollup".
Of course, in theory, it is doable, but the code will be harder to maintain
in the future. Extra Project will be collapsed by optimizer. Thus, in analyzer,
I just introduce the extra Project.
I am writing unit test cases. Will try to deliver them ASAP.
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