Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/4764#issuecomment-91093315
As a very rough sketch (this is totally untested and I'm probably missing
cases), I'd hope the solution could look something like the following:
```scala
object OptimizeSimpleDistincts extends Rule[LogicalPlan] {
def apply(plan: LogicalPlan): LogicalPlan = plan transform {
case Aggregate(Nil, Seq(agg), c) =>
val rewritten = agg transform {
case CountDistinct(Seq(c)) => Count(c)
case SumDistinct(c) => Sum(c)
}
if (rewritten != agg) {
Aggregate(Nil, rewritten.asInstanceOf[NamedExpression] :: Nil,
Distinct(c))
} else {
plan
}
}
}
```
With tests of course :) See `FilterPushdownSuite` for an example.
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