Modify comment and test cases.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala

@@ -882,7 +882,19 @@ case class Deduplicate(
   override def output: Seq[Attribute] = child.output
 }
 
-/** A logical plan for setting a barrier of analysis */
+/**
+ * A logical plan for setting a barrier of analysis.
+ *
+ * The SQL Analyzer goes through a whole query plan even most part of it is analyzed. This
+ * increases the time spent on query analysis for long pipelines in ML, especially.
+ *
+ * This logical plan wraps an analyzed logical plan to prevent it from analysis again. The barrier
+ * is applied to the analyzed logical plan in Dataset. It won't change the output of wrapped
+ * logical plan and just acts as a wrapper to hide it from analyzer. New operations on the dataset
+ * will be put on the barrier, so only the new nodes created will be analyzed.
+ *
+ * This analysis barrier will be removed at the end of analysis stage.
+ */
 case class AnalysisBarrier(child: LogicalPlan) extends LeafNode {
   override def output: Seq[Attribute] = child.output
   override def isStreaming: Boolean = child.isStreaming

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/plans/LogicalPlanSuite.scala

@@ -23,8 +23,8 @@ import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.types.IntegerType
 
 /**
- * This suite is used to test [[LogicalPlan]]'s `transformUp` plus analysis barrier and make sure
- * it can correctly skip sub-trees that have already been marked as analyzed.
+ * This suite is used to test [[LogicalPlan]]'s `transformUp/transformDown` plus analysis barrier
+ * and make sure it can correctly skip sub-trees that have already been analyzed.
  */
 class LogicalPlanSuite extends SparkFunSuite {
   private var invocationCount = 0
@@ -42,6 +42,10 @@ class LogicalPlanSuite extends SparkFunSuite {
     plan transformUp function
 
     assert(invocationCount === 1)
+
+    invocationCount = 0
+    plan transformDown function
+    assert(invocationCount === 1)
   }
 
   test("transformUp runs on operators recursively") {
@@ -50,6 +54,10 @@ class LogicalPlanSuite extends SparkFunSuite {
     plan transformUp function
 
     assert(invocationCount === 2)
+
+    invocationCount = 0
+    plan transformDown function
+    assert(invocationCount === 2)
   }
 
   test("transformUp skips all ready resolved plans wrapped in analysis barrier") {
@@ -58,6 +66,10 @@ class LogicalPlanSuite extends SparkFunSuite {
     plan transformUp function
 
     assert(invocationCount === 0)
+
+    invocationCount = 0
+    plan transformDown function
+    assert(invocationCount === 0)
   }
 
   test("transformUp skips partially resolved plans wrapped in analysis barrier") {
@@ -67,6 +79,10 @@ class LogicalPlanSuite extends SparkFunSuite {
     plan2 transformUp function
 
     assert(invocationCount === 1)
+
+    invocationCount = 0
+    plan2 transformDown function
+    assert(invocationCount === 1)
   }
 
   test("isStreaming") {