[SPARK-5404] [SQL] Update the default statistic number

By default, the statistic for logical plan with multiple children is quite aggressive, and those statistic are quite critical for the join optimization, hence we need to estimate the statistics as accurate as possible.

For `Union`, which has 2 children, and overwrite the default implementation by `adding` its children `byteInSize` instead of `multiplying`.
For `Expand`, which only has a single child, but it will grows the size, and we need to multiply its inflating factor.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #4914 from chenghao-intel/statistic and squashes the following commits:

d466bbc [Cheng Hao] Update the default statistic

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

@@ -81,6 +81,11 @@ case class Union(left: LogicalPlan, right: LogicalPlan) extends BinaryNode {
   override lazy val resolved =
     childrenResolved &&
     !left.output.zip(right.output).exists { case (l,r) => l.dataType != r.dataType }
+
+  override def statistics: Statistics = {
+    val sizeInBytes = left.statistics.sizeInBytes + right.statistics.sizeInBytes
+    Statistics(sizeInBytes = sizeInBytes)
+  }
 }
 
 case class Join(
@@ -174,7 +179,12 @@ case class Aggregate(
 case class Expand(
     projections: Seq[GroupExpression],
     output: Seq[Attribute],
-    child: LogicalPlan) extends UnaryNode
+    child: LogicalPlan) extends UnaryNode {
+  override def statistics: Statistics = {
+    val sizeInBytes = child.statistics.sizeInBytes * projections.length
+    Statistics(sizeInBytes = sizeInBytes)
+  }
+}
 
 trait GroupingAnalytics extends UnaryNode {
   self: Product =>