cloud-fan commented on code in PR #57826:
URL: https://github.com/apache/spark/pull/57826#discussion_r3759069016
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sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/ExpandBenchmark.scala:
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@@ -161,18 +182,63 @@ object ExpandBenchmark extends SqlBasedBenchmark {
|GROUP BY user_id""".stripMargin
}
+ /**
+ * Measures a traffic/BI "N-day active users" rollup (see
[[conditionalAggsQuery]])
+ * under all four combinations of the two optimizations that affect the
shared
+ * `daysSince` subexpression in the Expand branches:
+ * - base: rewriteCountDistinctConditional off, subexpression elimination
off
+ * -> 10 Expand branches (one per conditional COUNT(DISTINCT) plus one for
+ * the SUMs), daysSince evaluated 18 times per input row;
+ * - +rewrite: rewriteCountDistinctConditional on, CSE off -> the 9
+ * conditional COUNT(DISTINCT) collapse into one distinct group, so the
Review Comment:
**Nit:**
```suggestion
* conditional COUNT(DISTINCT) expressions collapse into one distinct
group, so the
```
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