[SPARK-4695][SQL] Get result using executeCollect

Using ```executeCollect``` to collect the result, because executeCollect is a custom implementation of collect in spark sql which better than rdd's collect

Author: wangfei <wangfei1@huawei.com>

Closes #3547 from scwf/executeCollect and squashes the following commits:

a5ab68e [wangfei] Revert "adding debug info"
a60d680 [wangfei] fix test failure
0db7ce8 [wangfei] adding debug info
184c594 [wangfei] using executeCollect instead collect

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala

@@ -377,7 +377,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
         command.executeCollect().map(_.head.toString)
 
       case other =>
-        val result: Seq[Seq[Any]] = toRdd.map(_.copy()).collect().toSeq
+        val result: Seq[Seq[Any]] = other.executeCollect().toSeq
         // We need the types so we can output struct field names
         val types = analyzed.output.map(_.dataType)
         // Reformat to match hive tab delimited output.
@@ -416,6 +416,8 @@ object HiveContext {
     case (bin: Array[Byte], BinaryType) => new String(bin, "UTF-8")
     case (decimal: Decimal, DecimalType()) =>  // Hive strips trailing zeros so use its toString
       HiveShim.createDecimal(decimal.toBigDecimal.underlying()).toString
+    case (decimal: BigDecimal, DecimalType()) =>
+      HiveShim.createDecimal(decimal.underlying()).toString
     case (other, tpe) if primitiveTypes contains tpe => other.toString
   }