[SPARK-14191][SQL] Remove invalid Expand operator constraints

`Expand` operator now uses its child plan's constraints as its valid constraints (i.e., the base of constraints). This is not correct because `Expand` will set its group by attributes to null values. So the nullability of these attributes should be true.

E.g., for an `Expand` operator like:

    val input = LocalRelation('a.int, 'b.int, 'c.int).where('c.attr > 10 && 'a.attr < 5 && 'b.attr > 2)
    Expand(
      Seq(
        Seq('c, Literal.create(null, StringType), 1),
        Seq('c, 'a, 2)),
      Seq('c, 'a, 'gid.int),
      Project(Seq('a, 'c), input))

The `Project` operator has the constraints `IsNotNull('a)`, `IsNotNull('b)` and `IsNotNull('c)`. But the `Expand` should not have `IsNotNull('a)` in its constraints.

This PR is the first step for this issue and remove invalid constraints of `Expand` operator.

A test is added to `ConstraintPropagationSuite`.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #11995 from viirya/fix-expand-constraints.

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

@@ -519,7 +519,6 @@ case class Expand(
     projections: Seq[Seq[Expression]],
     output: Seq[Attribute],
     child: LogicalPlan) extends UnaryNode {
-
   override def references: AttributeSet =
     AttributeSet(projections.flatten.flatMap(_.references))
 
@@ -527,6 +526,10 @@ case class Expand(
     val sizeInBytes = super.statistics.sizeInBytes * projections.length
     Statistics(sizeInBytes = sizeInBytes)
   }
+
+  // This operator can reuse attributes (for example making them null when doing a roll up) so
+  // the contraints of the child may no longer be valid.
+  override protected def validConstraints: Set[Expression] = Set.empty[Expression]
 }
 
 /**

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

@@ -88,6 +88,33 @@ class ConstraintPropagationSuite extends SparkFunSuite {
         IsNotNull(resolveColumn(aliasedRelation.analyze, "a")))))
   }
 
+  test("propagating constraints in expand") {
+    val tr = LocalRelation('a.int, 'b.int, 'c.int)
+
+    assert(tr.analyze.constraints.isEmpty)
+
+    // We add IsNotNull constraints for 'a, 'b and 'c into LocalRelation
+    // by creating notNullRelation.
+    val notNullRelation = tr.where('c.attr > 10 && 'a.attr < 5 && 'b.attr > 2)
+    verifyConstraints(notNullRelation.analyze.constraints,
+      ExpressionSet(Seq(resolveColumn(notNullRelation.analyze, "c") > 10,
+        IsNotNull(resolveColumn(notNullRelation.analyze, "c")),
+        resolveColumn(notNullRelation.analyze, "a") < 5,
+        IsNotNull(resolveColumn(notNullRelation.analyze, "a")),
+        resolveColumn(notNullRelation.analyze, "b") > 2,
+        IsNotNull(resolveColumn(notNullRelation.analyze, "b")))))
+
+    val expand = Expand(
+          Seq(
+            Seq('c, Literal.create(null, StringType), 1),
+            Seq('c, 'a, 2)),
+          Seq('c, 'a, 'gid.int),
+          Project(Seq('a, 'c),
+            notNullRelation))
+    verifyConstraints(expand.analyze.constraints,
+      ExpressionSet(Seq.empty[Expression]))
+  }
+
   test("propagating constraints in aliases") {
     val tr = LocalRelation('a.int, 'b.string, 'c.int)