[SPARK-9154][SQL] Rename formatString to format_string.

Also make format_string the canonical form, rather than printf.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -168,7 +168,7 @@ object FunctionRegistry {
     expression[StringLocate]("locate"),
     expression[StringLPad]("lpad"),
     expression[StringTrimLeft]("ltrim"),
-    expression[StringFormat]("printf"),
+    expression[FormatString]("printf"),
     expression[StringRPad]("rpad"),
     expression[StringRepeat]("repeat"),
     expression[StringReverse]("reverse"),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringOperations.scala

@@ -526,29 +526,26 @@ case class StringRPad(str: Expression, len: Expression, pad: Expression)
 /**
  * Returns the input formatted according do printf-style format strings
  */
-case class StringFormat(children: Expression*) extends Expression with ImplicitCastInputTypes {
+case class FormatString(children: Expression*) extends Expression with ImplicitCastInputTypes {
 
-  require(children.nonEmpty, "printf() should take at least 1 argument")
+  require(children.nonEmpty, "format_string() should take at least 1 argument")
 
   override def foldable: Boolean = children.forall(_.foldable)
   override def nullable: Boolean = children(0).nullable
   override def dataType: DataType = StringType
-  private def format: Expression = children(0)
-  private def args: Seq[Expression] = children.tail
 
   override def inputTypes: Seq[AbstractDataType] =
     StringType :: List.fill(children.size - 1)(AnyDataType)
 
-
   override def eval(input: InternalRow): Any = {
-    val pattern = format.eval(input)
+    val pattern = children(0).eval(input)
     if (pattern == null) {
       null
     } else {
       val sb = new StringBuffer()
       val formatter = new java.util.Formatter(sb, Locale.US)
 
-      val arglist = args.map(_.eval(input).asInstanceOf[AnyRef])
+      val arglist = children.tail.map(_.eval(input).asInstanceOf[AnyRef])
       formatter.format(pattern.asInstanceOf[UTF8String].toString, arglist: _*)
 
       UTF8String.fromString(sb.toString)
@@ -591,7 +588,7 @@ case class StringFormat(children: Expression*) extends Expression with ImplicitC
      """
   }
 
-  override def prettyName: String = "printf"
+  override def prettyName: String = "format_string"
 }
 
 /**

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/StringExpressionsSuite.scala

@@ -351,16 +351,16 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("FORMAT") {
-    checkEvaluation(StringFormat(Literal("aa%d%s"), Literal(123), Literal("a")), "aa123a")
-    checkEvaluation(StringFormat(Literal("aa")), "aa", create_row(null))
-    checkEvaluation(StringFormat(Literal("aa%d%s"), Literal(123), Literal("a")), "aa123a")
-    checkEvaluation(StringFormat(Literal("aa%d%s"), 12, "cc"), "aa12cc")
+    checkEvaluation(FormatString(Literal("aa%d%s"), Literal(123), Literal("a")), "aa123a")
+    checkEvaluation(FormatString(Literal("aa")), "aa", create_row(null))
+    checkEvaluation(FormatString(Literal("aa%d%s"), Literal(123), Literal("a")), "aa123a")
+    checkEvaluation(FormatString(Literal("aa%d%s"), 12, "cc"), "aa12cc")
 
-    checkEvaluation(StringFormat(Literal.create(null, StringType), 12, "cc"), null)
+    checkEvaluation(FormatString(Literal.create(null, StringType), 12, "cc"), null)
     checkEvaluation(
-      StringFormat(Literal("aa%d%s"), Literal.create(null, IntegerType), "cc"), "aanullcc")
+      FormatString(Literal("aa%d%s"), Literal.create(null, IntegerType), "cc"), "aanullcc")
     checkEvaluation(
-      StringFormat(Literal("aa%d%s"), 12, Literal.create(null, StringType)), "aa12null")
+      FormatString(Literal("aa%d%s"), 12, Literal.create(null, StringType)), "aa12null")
   }
 
   test("INSTR") {

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

@@ -1742,26 +1742,14 @@ object functions {
   def rtrim(e: Column): Column = StringTrimRight(e.expr)
 
   /**
-   * Format strings in printf-style.
+   * Formats the arguments in printf-style and returns the result as a string column.
    *
    * @group string_funcs
    * @since 1.5.0
    */
   @scala.annotation.varargs
-  def formatString(format: Column, arguments: Column*): Column = {
-    StringFormat((format +: arguments).map(_.expr): _*)
-  }
-
-  /**
-   * Format strings in printf-style.
-   * NOTE: `format` is the string value of the formatter, not column name.
-   *
-   * @group string_funcs
-   * @since 1.5.0
-   */
-  @scala.annotation.varargs
-  def formatString(format: String, arguNames: String*): Column = {
-    StringFormat(lit(format).expr +: arguNames.map(Column(_).expr): _*)
+  def format_string(format: String, arguments: Column*): Column = {
+    FormatString((lit(format) +: arguments).map(_.expr): _*)
   }
 
   /**

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

@@ -125,18 +125,14 @@ class StringFunctionsSuite extends QueryTest {
   test("string formatString function") {
     val df = Seq(("aa%d%s", 123, "cc")).toDF("a", "b", "c")
 
-    checkAnswer(
-      df.select(formatString("aa%d%s", "b", "c")),
-      Row("aa123cc"))
-
     checkAnswer(
       df.selectExpr("printf(a, b, c)"),
       Row("aa123cc"))
 
     val df2 = Seq(("aa%d%s".getBytes, 123, "cc")).toDF("a", "b", "c")
 
     checkAnswer(
-      df2.select(formatString($"a", $"b", $"c"), formatString("aa%d%s", "b", "c")),
+      df2.select(format_string("a", $"b", $"c"), format_string("aa%d%s", $"b", $"c")),
       Row("aa123cc", "aa123cc"))
 
     checkAnswer(