[SPARK-12105] [SQL] add convenient show functions

Author: Jean-Baptiste Onofré <[email protected]>

Closes #10130 from jbonofre/SPARK-12105.

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -160,17 +160,24 @@ class DataFrame private[sql](
     }
   }
 
+  /**
+    * Compose the string representing rows for output
+    */
+  def showString(): String = {
+    showString(20)
+  }
+
   /**
    * Compose the string representing rows for output
-   * @param _numRows Number of rows to show
+   * @param numRows Number of rows to show
    * @param truncate Whether truncate long strings and align cells right
    */
-  private[sql] def showString(_numRows: Int, truncate: Boolean = true): String = {
-    val numRows = _numRows.max(0)
+  def showString(numRows: Int, truncate: Boolean = true): String = {
+    val _numRows = numRows.max(0)
     val sb = new StringBuilder
-    val takeResult = take(numRows + 1)
-    val hasMoreData = takeResult.length > numRows
-    val data = takeResult.take(numRows)
+    val takeResult = take(_numRows + 1)
+    val hasMoreData = takeResult.length > _numRows
+    val data = takeResult.take(_numRows)
     val numCols = schema.fieldNames.length
 
     // For array values, replace Seq and Array with square brackets
@@ -224,10 +231,10 @@ class DataFrame private[sql](
 
     sb.append(sep)
 
-    // For Data that has more than "numRows" records
+    // For Data that has more than "_numRows" records
     if (hasMoreData) {
-      val rowsString = if (numRows == 1) "row" else "rows"
-      sb.append(s"only showing top $numRows $rowsString\n")
+      val rowsString = if (_numRows == 1) "row" else "rows"
+      sb.append(s"only showing top $_numRows $rowsString\n")
     }
 
     sb.toString()