[SPARK-7982][SQL] DataFrame.stat.crosstab should use 0 instead of null for pairs that don't appear

Author: Reynold Xin <rxin@databricks.com>

Closes #6566 from rxin/crosstab and squashes the following commits:

e0ace1c [Reynold Xin] [SPARK-7982][SQL] DataFrame.stat.crosstab should use 0 instead of null for pairs that don't appear

sql/core/src/main/scala/org/apache/spark/sql/execution/stat/StatFunctions.scala

@@ -18,7 +18,7 @@
 package org.apache.spark.sql.execution.stat
 
 import org.apache.spark.Logging
-import org.apache.spark.sql.{Column, DataFrame}
+import org.apache.spark.sql.{Row, Column, DataFrame}
 import org.apache.spark.sql.catalyst.expressions.{GenericMutableRow, Cast}
 import org.apache.spark.sql.catalyst.plans.logical.LocalRelation
 import org.apache.spark.sql.functions._
@@ -116,7 +116,10 @@ private[sql] object StatFunctions extends Logging {
       s"exceed 1e4. Currently $columnSize")
     val table = counts.groupBy(_.get(0)).map { case (col1Item, rows) =>
       val countsRow = new GenericMutableRow(columnSize + 1)
-      rows.foreach { row =>
+      rows.foreach { (row: Row) =>
+        // row.get(0) is column 1
+        // row.get(1) is column 2
+        // row.get(3) is the frequency
         countsRow.setLong(distinctCol2.get(row.get(1)).get + 1, row.getLong(2))
       }
       // the value of col1 is the first value, the rest are the counts
@@ -126,6 +129,6 @@ private[sql] object StatFunctions extends Logging {
     val headerNames = distinctCol2.map(r => StructField(r._1.toString, LongType)).toSeq
     val schema = StructType(StructField(tableName, StringType) +: headerNames)
 
-    new DataFrame(df.sqlContext, LocalRelation(schema.toAttributes, table))
+    new DataFrame(df.sqlContext, LocalRelation(schema.toAttributes, table)).na.fill(0.0)
   }
 }

sql/core/src/test/scala/org/apache/spark/sql/DataFrameStatSuite.scala

@@ -74,10 +74,10 @@ class DataFrameStatSuite extends SparkFunSuite  {
     val rows: Array[Row] = crosstab.collect().sortBy(_.getString(0))
     assert(rows(0).get(0).toString === "0")
     assert(rows(0).getLong(1) === 2L)
-    assert(rows(0).get(2) === null)
+    assert(rows(0).get(2) === 0L)
     assert(rows(1).get(0).toString === "1")
     assert(rows(1).getLong(1) === 1L)
-    assert(rows(1).get(2) === null)
+    assert(rows(1).get(2) === 0L)
     assert(rows(2).get(0).toString === "2")
     assert(rows(2).getLong(1) === 2L)
     assert(rows(2).getLong(2) === 1L)