[SPARK-27279][SQL] Reuse subquery should compare child plan of SubqueryExec

## What changes were proposed in this pull request?

For now, `ReuseSubquery` in Spark compares two subqueries at `SubqueryExec` level, which invalidates the `ReuseSubquery` rule. This pull request fixes this, and add a configuration key for subquery reuse exclusively.

## How was this patch tested?

add a unit test.

Closes #24214 from adrian-wang/reuse.

Authored-by: Daoyuan Wang <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

Author: adrian-wang

sql/core/src/main/scala/org/apache/spark/sql/execution/basicPhysicalOperators.scala

@@ -674,6 +674,8 @@ case class SubqueryExec(name: String, child: SparkPlan) extends UnaryExecNode {
 
   override def outputOrdering: Seq[SortOrder] = child.outputOrdering
 
+  override def doCanonicalize(): SparkPlan = child.canonicalized
+
   @transient
   private lazy val relationFuture: Future[Array[InternalRow]] = {
     // relationFuture is used in "doExecute". Therefore we can get the execution id correctly here.

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -25,7 +25,7 @@ import java.util.concurrent.atomic.AtomicBoolean
 import org.apache.spark.{AccumulatorSuite, SparkException}
 import org.apache.spark.scheduler.{SparkListener, SparkListenerJobStart}
 import org.apache.spark.sql.catalyst.util.StringUtils
-import org.apache.spark.sql.execution.aggregate
+import org.apache.spark.sql.execution.{aggregate, ScalarSubquery, SubqueryExec}
 import org.apache.spark.sql.execution.aggregate.{HashAggregateExec, SortAggregateExec}
 import org.apache.spark.sql.execution.datasources.FilePartition
 import org.apache.spark.sql.execution.joins.{BroadcastHashJoinExec, CartesianProductExec, SortMergeJoinExec}
@@ -113,6 +113,33 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
     }
   }
 
+  test("Reuse Subquery") {
+    Seq(true, false).foreach { reuse =>
+      withSQLConf(SQLConf.SUBQUERY_REUSE_ENABLED.key -> reuse.toString) {
+        val df = sql(
+          """
+            |SELECT (SELECT avg(key) FROM testData) + (SELECT avg(key) FROM testData)
+            |FROM testData
+            |LIMIT 1
+          """.stripMargin)
+
+        import scala.collection.mutable.ArrayBuffer
+        val subqueries = ArrayBuffer[SubqueryExec]()
+        df.queryExecution.executedPlan.transformAllExpressions {
+          case s @ ScalarSubquery(plan: SubqueryExec, _) =>
+            subqueries += plan
+            s
+        }
+
+        if (reuse) {
+          assert(subqueries.distinct.size == 1, "Subquery reusing not working correctly")
+        } else {
+          assert(subqueries.distinct.size == 2, "There should be 2 subqueries when not reusing")
+        }
+      }
+    }
+  }
+
   test("SPARK-6743: no columns from cache") {
     Seq(
       (83, 0, 38),