update comments.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala

@@ -81,7 +81,8 @@ abstract class QueryPlan[PlanType <: QueryPlan[PlanType]] extends TreeNode[PlanT
     case _ => Seq.empty[Attribute]
   }
 
-  // Collect aliases from expressions, so we may avoid producing recursive constraints.
+  // Collect aliases from expressions of the whole tree rooted by the current QueryPlan node, so
+  // we may avoid producing recursive constraints.
   private lazy val aliasMap: AttributeMap[Expression] = AttributeMap(
     expressions.collect {
       case a: Alias => (a.toAttribute, a.child)

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala

@@ -173,7 +173,11 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
     val t1 = testRelation.subquery('t1)
     val t2 = testRelation.subquery('t2)
 
+    // We should prevent `Coalese(a, b)` from recursively creating complicated constraints through
+    // the constraint inference procedure.
     val originalQuery = t1.select('a, 'b.as('d), Coalesce(Seq('a, 'b)).as('int_col))
+      // We hide an `Alias` inside the child's child's expressions, to cover the situation reported
+      // in [SPARK-20700].
       .select('int_col, 'd, 'a).as("t")
       .join(t2, Inner,
         Some("t.a".attr === "t2.a".attr