[GitHub] [spark] JoshRosen commented on a change in pull request #24505: [SPARK-27607][SQL] Improve Row.toString performance

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JoshRosen commented on a change in pull request #24505: [SPARK-27607][SQL] Improve Row.toString performance
URL: https://github.com/apache/spark/pull/24505#discussion_r280097784
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/Row.scala
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 @@ -465,16 +465,34 @@ trait Row extends Serializable {
   }
 
   /** Displays all elements of this sequence in a string (without a separator). */
-  def mkString: String = toSeq.mkString
+  def mkString: String = mkString("")
 
   /** Displays all elements of this sequence in a string using a separator string. */
-  def mkString(sep: String): String = toSeq.mkString(sep)
+  def mkString(sep: String): String = mkString("", sep, "")
 
   /**
    * Displays all elements of this traversable or iterator in a string using
    * start, end, and separator strings.
    */
-  def mkString(start: String, sep: String, end: String): String = toSeq.mkString(start, sep, end)
+  def mkString(start: String, sep: String, end: String): String = {
+    var first = true
+    var i = 0
+    val n = length
+    val builder = new StringBuilder
+    builder.append(start)
+    while (i < n) {
+      if (first) {
 
 Review comment:
   Could this be an `i ==0` instead, eliminating the need to assign another variable? If I recall that might help with JIT (but not 100 percent sure)

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