Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/9406#discussion_r44203475
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Utils.scala
---
@@ -213,3 +216,178 @@ object Utils {
case other => None
}
}
+
+/**
+ * This rule rewrites an aggregate query with multiple distinct clauses
into an expanded double
+ * aggregation in which the regular aggregation expressions and every
distinct clause is aggregated
+ * in a separate group. The results are then combined in a second
aggregate.
+ *
+ * TODO Expression cannocalization
+ * TODO Eliminate foldable expressions from distinct clauses.
+ * TODO This eliminates all distinct expressions. We could safely pass one
to the aggregate
+ * operator. Perhaps this is a good thing? It is much simpler to plan
later on...
--- End diff --
These are all optimizations. The last one is perhaps a question for @yhuai:
we have a choice to rewrite all distinct expressions.
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