squito commented on a change in pull request #28885:
URL: https://github.com/apache/spark/pull/28885#discussion_r465997277
##########
File path:
sql/core/src/test/scala/org/apache/spark/sql/execution/ExchangeSuite.scala
##########
@@ -156,4 +158,46 @@ class ExchangeSuite extends SparkPlanTest with
SharedSparkSession {
val projection2 = cached.select("_1", "_3").queryExecution.executedPlan
assert(!projection1.sameResult(projection2))
}
+
+ test("Exchange reuse across the whole plan") {
+ val df = sql(
+ """
+ |SELECT
+ | (SELECT max(a.key) FROM testData AS a JOIN testData AS b ON b.key =
a.key),
+ | a.key
+ |FROM testData AS a
+ |JOIN testData AS b ON b.key = a.key
+ """.stripMargin)
+
+ val plan = df.queryExecution.executedPlan
+
+ val exchangeIds = plan.collectWithSubqueries { case e: Exchange => e.id }
+ val reusedExchangeIds = plan.collectWithSubqueries {
+ case re: ReusedExchangeExec => re.child.id
+ }
+
+ assert(exchangeIds.size == 2, "Whole plan exchange reusing not working
correctly")
+ assert(reusedExchangeIds.size == 3, "Whole plan exchange reusing not
working correctly")
+ assert(reusedExchangeIds.forall(exchangeIds.contains(_)),
+ "ReusedExchangeExec should reuse an existing exchange")
+
+ val df2 = sql(
Review comment:
Sorry I am getting back to this so late.
So, I see what you are saying, but this sounds super brittle. Again, I'm
not an expert here, but I wouldn't have expected rules to be so specific to
their ordering. What if someone else adds another rule which they decide also
has to be last, for some other reason? The abstraction suggests the rules
should operate relatively independently.
It would seem better to have a mechanism in `transform()` etc. functions to
deal with this kind of thing. Eg. there already is a call to `copyTagsFrom()`,
couldn't there be something similar there to keep the references between
"reused" nodes intact?
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