Github user maropu commented on a diff in the pull request:
https://github.com/apache/spark/pull/21857#discussion_r204763415
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -1275,6 +1276,64 @@ object ReplaceExceptWithAntiJoin extends
Rule[LogicalPlan] {
}
}
+/**
+ * Replaces logical [[ExceptAll]] operator using a combination of Union,
Aggregate
+ * and Generate operator.
+ *
+ * Input Query :
+ * {{{
+ * SELECT c1 FROM ut1 EXCEPT ALL SELECT c1 FROM ut2
+ * }}}
+ *
+ * Rewritten Query:
+ * {{{
+ * SELECT c1
+ * FROM (
+ * SELECT replicate_rows(sum_val, c1) AS (sum_val, c1)
+ * FROM (
+ * SELECT c1, cnt, sum_val
+ * FROM (
+ * SELECT c1, sum(vcol) AS sum_val
+ * FROM (
+ * SELECT 1L as vcol, c1 FROM ut1
+ * UNION ALL
+ * SELECT -1L as vcol, c1 FROM ut2
+ * ) AS union_all
+ * GROUP BY union_all.c1
+ * )
+ * WHERE sum_val > 0
+ * )
+ * )
+ * }}}
+ */
+
+object RewriteExcepAll extends Rule[LogicalPlan] {
--- End diff --
We need tests for the `RewriteExceptAll rule (you can check
`RewriteDistinctAggregatesSuite` as a reference).
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