[SQL] When creating partitioned table scan, explicitly create UnionRDD.

Otherwise, it will cause stack overflow when there are many partitions.

Author: Yin Huai <yhuai@databricks.com>

Closes #6162 from yhuai/partitionUnionedRDD and squashes the following commits:

fa016d8 [Yin Huai] Explicitly create UnionRDD.

sql/core/src/main/scala/org/apache/spark/sql/sources/DataSourceStrategy.scala

@@ -21,7 +21,7 @@ import org.apache.hadoop.fs.Path
 
 import org.apache.spark.Logging
 import org.apache.spark.deploy.SparkHadoopUtil
-import org.apache.spark.rdd.RDD
+import org.apache.spark.rdd.{UnionRDD, RDD}
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.sql.catalyst.expressions._
@@ -169,9 +169,12 @@ private[sql] object DataSourceStrategy extends Strategy with Logging {
       scan.execute()
     }
 
-    val unionedRows = perPartitionRows.reduceOption(_ ++ _).getOrElse {
-      relation.sqlContext.emptyResult
-    }
+    val unionedRows =
+      if (perPartitionRows.length == 0) {
+        relation.sqlContext.emptyResult
+      } else {
+        new UnionRDD(relation.sqlContext.sparkContext, perPartitionRows)
+      }
 
     createPhysicalRDD(logicalRelation.relation, output, unionedRows)
   }