[SPARK-21103][SQL] QueryPlanConstraints should be part of LogicalPlan

## What changes were proposed in this pull request?
QueryPlanConstraints should be part of LogicalPlan, rather than QueryPlan, since the constraint framework is only used for query plan rewriting and not for physical planning.

## How was this patch tested?
Should be covered by existing tests, since it is a simple refactoring.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#18310 from rxin/SPARK-21103.

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

@@ -22,10 +22,7 @@ import org.apache.spark.sql.catalyst.trees.TreeNode
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types.{DataType, StructType}
 
-abstract class QueryPlan[PlanType <: QueryPlan[PlanType]]
-  extends TreeNode[PlanType]
-  with QueryPlanConstraints[PlanType] {
-
+abstract class QueryPlan[PlanType <: QueryPlan[PlanType]] extends TreeNode[PlanType] {
   self: PlanType =>
 
   def conf: SQLConf = SQLConf.get

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

@@ -27,7 +27,7 @@ import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types.StructType
 
 
-abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
+abstract class LogicalPlan extends QueryPlan[LogicalPlan] with QueryPlanConstraints with Logging {
 
   private var _analyzed: Boolean = false
 

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

@@ -15,12 +15,12 @@
  * limitations under the License.
  */
 
-package org.apache.spark.sql.catalyst.plans
+package org.apache.spark.sql.catalyst.plans.logical
 
 import org.apache.spark.sql.catalyst.expressions._
 
 
-trait QueryPlanConstraints[PlanType <: QueryPlan[PlanType]] { self: QueryPlan[PlanType] =>
+trait QueryPlanConstraints { self: LogicalPlan =>
 
   /**
    * An [[ExpressionSet]] that contains invariants about the rows output by this operator. For
@@ -99,7 +99,8 @@ trait QueryPlanConstraints[PlanType <: QueryPlan[PlanType]] { self: QueryPlan[Pl
   private lazy val aliasMap: AttributeMap[Expression] = AttributeMap(
     expressions.collect {
       case a: Alias => (a.toAttribute, a.child)
-    } ++ children.flatMap(_.asInstanceOf[QueryPlanConstraints[PlanType]].aliasMap))
+    } ++ children.flatMap(_.asInstanceOf[QueryPlanConstraints].aliasMap))
+    // Note: the explicit cast is necessary, since Scala compiler fails to infer the type.
 
   /**
    * Infers an additional set of constraints from a given set of equality constraints.