Github user liancheng commented on the pull request:
https://github.com/apache/spark/pull/11283#issuecomment-192624981
A final thought about this PR:
For SQL generation for grouping sets, we have to depend on a set of
implicit assumptions related to implementation details of specific analysis
rules, which makes the implementation tend to be fragile. I think maybe we can
add an auxiliary logical plan operator `AnnotatePlan` and a expression
`AnnotateExpression`, which can be used to annotate a sub-tree of a logical
plan / expression to indicate their original form. These nodes should be erased
by the optimizer.
Haven't thought thoroughly about this, but with the help of these
annotations, I'd expect it to be easier to recognize patterns corresponding to
plans / expressions like grouping set, grouping, multi-distinct aggregation
etc..
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