From de38cf895f12f4433b10702fab12efaa179dce27 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 12:42:41 +0100
Subject: [PATCH 01/13] Use append function instead of Writer

---
 .../spark/sql/catalyst/trees/TreeNode.scala   | 26 +++++++--------
 .../spark/sql/execution/QueryExecution.scala  | 32 +++++++++----------
 .../sql/execution/WholeStageCodegenExec.scala |  8 ++---
 3 files changed, 33 insertions(+), 33 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
index 21e59bbd283e..d46b8b71e744 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
@@ -483,7 +483,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       maxFields: Int = SQLConf.get.maxToStringFields): String = {
     val writer = new StringBuilderWriter()
     try {
-      treeString(writer, verbose, addSuffix, maxFields)
+      treeString(writer.write, verbose, addSuffix, maxFields)
       writer.toString
     } finally {
       writer.close()
@@ -491,11 +491,11 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   }
 
   def treeString(
-      writer: Writer,
+      append: String => Unit,
       verbose: Boolean,
       addSuffix: Boolean,
       maxFields: Int): Unit = {
-    generateTreeString(0, Nil, writer, verbose, "", addSuffix, maxFields)
+    generateTreeString(0, Nil, append, verbose, "", addSuffix, maxFields)
   }
 
   /**
@@ -558,7 +558,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   def generateTreeString(
       depth: Int,
       lastChildren: Seq[Boolean],
-      writer: Writer,
+      append: String => Unit,
       verbose: Boolean,
       prefix: String = "",
       addSuffix: Boolean = false,
@@ -566,9 +566,9 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
 
     if (depth > 0) {
       lastChildren.init.foreach { isLast =>
-        writer.write(if (isLast) "   " else ":  ")
+        append(if (isLast) "   " else ":  ")
       }
-      writer.write(if (lastChildren.last) "+- " else ":- ")
+      append(if (lastChildren.last) "+- " else ":- ")
     }
 
     val str = if (verbose) {
@@ -576,24 +576,24 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
     } else {
       simpleString(maxFields)
     }
-    writer.write(prefix)
-    writer.write(str)
-    writer.write("\n")
+    append(prefix)
+    append(str)
+    append("\n")
 
     if (innerChildren.nonEmpty) {
       innerChildren.init.foreach(_.generateTreeString(
-        depth + 2, lastChildren :+ children.isEmpty :+ false, writer, verbose,
+        depth + 2, lastChildren :+ children.isEmpty :+ false, append, verbose,
         addSuffix = addSuffix, maxFields = maxFields))
       innerChildren.last.generateTreeString(
-        depth + 2, lastChildren :+ children.isEmpty :+ true, writer, verbose,
+        depth + 2, lastChildren :+ children.isEmpty :+ true, append, verbose,
         addSuffix = addSuffix, maxFields = maxFields)
     }
 
     if (children.nonEmpty) {
       children.init.foreach(_.generateTreeString(
-        depth + 1, lastChildren :+ false, writer, verbose, prefix, addSuffix, maxFields))
+        depth + 1, lastChildren :+ false, append, verbose, prefix, addSuffix, maxFields))
       children.last.generateTreeString(
-        depth + 1, lastChildren :+ true, writer, verbose, prefix, addSuffix, maxFields)
+        depth + 1, lastChildren :+ true, append, verbose, prefix, addSuffix, maxFields)
     }
   }
 
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index 9b8d2e830867..b75ac09fa54c 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -202,34 +202,34 @@ class QueryExecution(
       """.stripMargin.trim
   }
 
-  private def writeOrError(writer: Writer)(f: Writer => Unit): Unit = {
-    try f(writer)
+  private def appendOrError(append: String => Unit)(f: (String => Unit) => Unit): Unit = {
+    try f(append)
     catch {
-      case e: AnalysisException => writer.write(e.toString)
+      case e: AnalysisException => append(e.toString)
     }
   }
 
-  private def writePlans(writer: Writer, maxFields: Int): Unit = {
+  private def writePlans(append: String => Unit, maxFields: Int): Unit = {
     val (verbose, addSuffix) = (true, false)
 
-    writer.write("== Parsed Logical Plan ==\n")
-    writeOrError(writer)(logical.treeString(_, verbose, addSuffix, maxFields))
-    writer.write("\n== Analyzed Logical Plan ==\n")
+    append("== Parsed Logical Plan ==\n")
+    appendOrError(append)(logical.treeString(_, verbose, addSuffix, maxFields))
+    append("\n== Analyzed Logical Plan ==\n")
     val analyzedOutput = stringOrError(truncatedString(
       analyzed.output.map(o => s"${o.name}: ${o.dataType.simpleString}"), ", ", maxFields))
-    writer.write(analyzedOutput)
-    writer.write("\n")
-    writeOrError(writer)(analyzed.treeString(_, verbose, addSuffix, maxFields))
-    writer.write("\n== Optimized Logical Plan ==\n")
-    writeOrError(writer)(optimizedPlan.treeString(_, verbose, addSuffix, maxFields))
-    writer.write("\n== Physical Plan ==\n")
-    writeOrError(writer)(executedPlan.treeString(_, verbose, addSuffix, maxFields))
+    append(analyzedOutput)
+    append("\n")
+    appendOrError(append)(analyzed.treeString(_, verbose, addSuffix, maxFields))
+    append("\n== Optimized Logical Plan ==\n")
+    appendOrError(append)(optimizedPlan.treeString(_, verbose, addSuffix, maxFields))
+    append("\n== Physical Plan ==\n")
+    appendOrError(append)(executedPlan.treeString(_, verbose, addSuffix, maxFields))
   }
 
   override def toString: String = withRedaction {
     val writer = new StringBuilderWriter()
     try {
-      writePlans(writer, SQLConf.get.maxToStringFields)
+      writePlans(writer.write, SQLConf.get.maxToStringFields)
       writer.toString
     } finally {
       writer.close()
@@ -290,7 +290,7 @@ class QueryExecution(
       val writer = new BufferedWriter(new OutputStreamWriter(fs.create(filePath)))
 
       try {
-        writePlans(writer, maxFields)
+        writePlans(writer.write, maxFields)
         writer.write("\n== Whole Stage Codegen ==\n")
         org.apache.spark.sql.execution.debug.writeCodegen(writer, executedPlan)
       } finally {
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
index f4927dedabe5..3b0a99669ccd 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
@@ -493,7 +493,7 @@ case class InputAdapter(child: SparkPlan) extends UnaryExecNode with InputRDDCod
   override def generateTreeString(
       depth: Int,
       lastChildren: Seq[Boolean],
-      writer: Writer,
+      append: String => Unit,
       verbose: Boolean,
       prefix: String = "",
       addSuffix: Boolean = false,
@@ -501,7 +501,7 @@ case class InputAdapter(child: SparkPlan) extends UnaryExecNode with InputRDDCod
     child.generateTreeString(
       depth,
       lastChildren,
-      writer,
+      append,
       verbose,
       prefix = "",
       addSuffix = false,
@@ -777,7 +777,7 @@ case class WholeStageCodegenExec(child: SparkPlan)(val codegenStageId: Int)
   override def generateTreeString(
       depth: Int,
       lastChildren: Seq[Boolean],
-      writer: Writer,
+      append: String => Unit,
       verbose: Boolean,
       prefix: String = "",
       addSuffix: Boolean = false,
@@ -785,7 +785,7 @@ case class WholeStageCodegenExec(child: SparkPlan)(val codegenStageId: Int)
     child.generateTreeString(
       depth,
       lastChildren,
-      writer,
+      append,
       verbose,
       s"*($codegenStageId) ",
       false,

From af5c9ed389f4d6caaba49969e48d966d9d976831 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 12:43:52 +0100
Subject: [PATCH 02/13] Fix typo

---
 .../scala/org/apache/spark/sql/execution/QueryExecution.scala   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index b75ac09fa54c..5ab6312b7e13 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -282,7 +282,7 @@ class QueryExecution(
     /**
      * Dumps debug information about query execution into the specified file.
      *
-     * @param maxFields maximim number of fields converted to string representation.
+     * @param maxFields maximum number of fields converted to string representation.
      */
     def toFile(path: String, maxFields: Int = Int.MaxValue): Unit = {
       val filePath = new Path(path)

From 0a36a26a8bdbace64be8260189475014cb1c0e27 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 13:20:10 +0100
Subject: [PATCH 03/13] StringRope on List

---
 .../spark/sql/catalyst/util/StringUtils.scala | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index bc861a805ce6..9912fa994294 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -87,4 +87,26 @@ object StringUtils {
     }
     funcNames.toSeq
   }
+
+  class StringRope {
+    private var list = List.empty[String]
+    private var length: Int = 0
+
+    def append(s: String): Unit = {
+      list = s :: list
+      length += s.length
+    }
+
+    override def toString: String = {
+      val buffer = new StringBuffer(length)
+      var reversed = list.reverse
+
+      while (!reversed.isEmpty) {
+        buffer.append(reversed.head)
+        reversed = reversed.tail
+      }
+
+      buffer.toString
+    }
+  }
 }

From 07ae72d952a239aef1b58e0ae3e1028292de293e Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 13:39:17 +0100
Subject: [PATCH 04/13] Tests for StringRope

---
 .../spark/sql/catalyst/util/StringUtils.scala       |  6 ++++--
 .../spark/sql/catalyst/util/StringUtilsSuite.scala  | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index 9912fa994294..1a0cc4d80f6e 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -93,8 +93,10 @@ object StringUtils {
     private var length: Int = 0
 
     def append(s: String): Unit = {
-      list = s :: list
-      length += s.length
+      if (s != null) {
+        list = s :: list
+        length += s.length
+      }
     }
 
     override def toString: String = {
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
index 78fee5135c3a..42be9980e95a 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
@@ -43,4 +43,17 @@ class StringUtilsSuite extends SparkFunSuite {
     assert(filterPattern(names, " a. ") === Seq("a1", "a2"))
     assert(filterPattern(names, " d* ") === Nil)
   }
+
+  test("string rope") {
+    def toRope(seq: String*): String = {
+      seq.foldLeft(new StringRope())((rope, s) => {rope.append(s); rope}).toString
+    }
+
+    assert(new StringRope().toString == "")
+    assert(toRope("") == "")
+    assert(toRope(null) == "")
+    assert(toRope("a") == "a")
+    assert(toRope("1", "2") == "12")
+    assert(toRope("abc", "\n", "123") == "abc\n123")
+  }
 }

From f621de72f6f3e623c6660e50302f2e6abb5557d7 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 13:54:16 +0100
Subject: [PATCH 05/13] Porting on StringRope

---
 .../spark/sql/catalyst/trees/TreeNode.scala   | 14 ++++------
 .../spark/sql/execution/QueryExecution.scala  | 17 +++++-------
 .../spark/sql/execution/debug/package.scala   | 26 +++++++------------
 3 files changed, 22 insertions(+), 35 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
index d46b8b71e744..202d1b527de6 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
@@ -17,13 +17,11 @@
 
 package org.apache.spark.sql.catalyst.trees
 
-import java.io.Writer
 import java.util.UUID
 
 import scala.collection.Map
 import scala.reflect.ClassTag
 
-import org.apache.commons.io.output.StringBuilderWriter
 import org.apache.commons.lang3.ClassUtils
 import org.json4s.JsonAST._
 import org.json4s.JsonDSL._
@@ -37,6 +35,7 @@ import org.apache.spark.sql.catalyst.errors._
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.JoinType
 import org.apache.spark.sql.catalyst.plans.physical.{BroadcastMode, Partitioning}
+import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
 import org.apache.spark.sql.catalyst.util.truncatedString
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types._
@@ -481,13 +480,10 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       verbose: Boolean,
       addSuffix: Boolean = false,
       maxFields: Int = SQLConf.get.maxToStringFields): String = {
-    val writer = new StringBuilderWriter()
-    try {
-      treeString(writer.write, verbose, addSuffix, maxFields)
-      writer.toString
-    } finally {
-      writer.close()
-    }
+    val rope = new StringRope()
+
+    treeString(rope.append, verbose, addSuffix, maxFields)
+    rope.toString
   }
 
   def treeString(
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index 5ab6312b7e13..c842efa0a0d0 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -17,11 +17,10 @@
 
 package org.apache.spark.sql.execution
 
-import java.io.{BufferedWriter, OutputStreamWriter, Writer}
+import java.io.{BufferedWriter, OutputStreamWriter}
 import java.nio.charset.StandardCharsets
 import java.sql.{Date, Timestamp}
 
-import org.apache.commons.io.output.StringBuilderWriter
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark.rdd.RDD
@@ -31,6 +30,7 @@ import org.apache.spark.sql.catalyst.analysis.UnsupportedOperationChecker
 import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, ReturnAnswer}
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.catalyst.util.DateTimeUtils
+import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
 import org.apache.spark.sql.catalyst.util.truncatedString
 import org.apache.spark.sql.execution.command.{DescribeTableCommand, ExecutedCommandExec, ShowTablesCommand}
 import org.apache.spark.sql.execution.exchange.{EnsureRequirements, ReuseExchange}
@@ -227,13 +227,10 @@ class QueryExecution(
   }
 
   override def toString: String = withRedaction {
-    val writer = new StringBuilderWriter()
-    try {
-      writePlans(writer.write, SQLConf.get.maxToStringFields)
-      writer.toString
-    } finally {
-      writer.close()
-    }
+    val rope = new StringRope()
+
+    writePlans(rope.append, SQLConf.get.maxToStringFields)
+    rope.toString
   }
 
   def stringWithStats: String = withRedaction {
@@ -292,7 +289,7 @@ class QueryExecution(
       try {
         writePlans(writer.write, maxFields)
         writer.write("\n== Whole Stage Codegen ==\n")
-        org.apache.spark.sql.execution.debug.writeCodegen(writer, executedPlan)
+        org.apache.spark.sql.execution.debug.writeCodegen(writer.write, executedPlan)
       } finally {
         writer.close()
       }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
index ae8197f617a2..d52d889318cf 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
@@ -17,13 +17,10 @@
 
 package org.apache.spark.sql.execution
 
-import java.io.Writer
 import java.util.Collections
 
 import scala.collection.JavaConverters._
 
-import org.apache.commons.io.output.StringBuilderWriter
-
 import org.apache.spark.internal.Logging
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql._
@@ -32,6 +29,7 @@ import org.apache.spark.sql.catalyst.expressions.Attribute
 import org.apache.spark.sql.catalyst.expressions.codegen.{CodeFormatter, CodegenContext, ExprCode}
 import org.apache.spark.sql.catalyst.plans.physical.Partitioning
 import org.apache.spark.sql.catalyst.trees.TreeNodeRef
+import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
 import org.apache.spark.sql.execution.streaming.{StreamExecution, StreamingQueryWrapper}
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.streaming.StreamingQuery
@@ -73,24 +71,20 @@ package object debug {
    * @return single String containing all WholeStageCodegen subtrees and corresponding codegen
    */
   def codegenString(plan: SparkPlan): String = {
-    val writer = new StringBuilderWriter()
+    val rope = new StringRope()
 
-    try {
-      writeCodegen(writer, plan)
-      writer.toString
-    } finally {
-      writer.close()
-    }
+    writeCodegen(rope.append, plan)
+    rope.toString
   }
 
-  def writeCodegen(writer: Writer, plan: SparkPlan): Unit = {
+  def writeCodegen(append: String => Unit, plan: SparkPlan): Unit = {
     val codegenSeq = codegenStringSeq(plan)
-    writer.write(s"Found ${codegenSeq.size} WholeStageCodegen subtrees.\n")
+    append(s"Found ${codegenSeq.size} WholeStageCodegen subtrees.\n")
     for (((subtree, code), i) <- codegenSeq.zipWithIndex) {
-      writer.write(s"== Subtree ${i + 1} / ${codegenSeq.size} ==\n")
-      writer.write(subtree)
-      writer.write("\nGenerated code:\n")
-      writer.write(s"${code}\n")
+      append(s"== Subtree ${i + 1} / ${codegenSeq.size} ==\n")
+      append(subtree)
+      append("\nGenerated code:\n")
+      append(s"${code}\n")
     }
   }
 

From ca4aed8f7ef02184afd5e3ef057b6babde4274dc Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sat, 29 Dec 2018 18:17:11 +0100
Subject: [PATCH 06/13] Rope other methods

---
 .../spark/sql/execution/QueryExecution.scala  | 33 ++++++++++++-------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index c842efa0a0d0..8d1c7f60db6f 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -108,10 +108,6 @@ class QueryExecution(
     ReuseExchange(sparkSession.sessionState.conf),
     ReuseSubquery(sparkSession.sessionState.conf))
 
-  protected def stringOrError[A](f: => A): String =
-    try f.toString catch { case e: AnalysisException => e.toString }
-
-
   /**
    * Returns the result as a hive compatible sequence of strings. This is used in tests and
    * `SparkSQLDriver` for CLI applications.
@@ -197,9 +193,14 @@ class QueryExecution(
   }
 
   def simpleString: String = withRedaction {
-    s"""== Physical Plan ==
-       |${stringOrError(executedPlan.treeString(verbose = false))}
-      """.stripMargin.trim
+    val rope = new StringRope()
+
+    rope.append("== Physical Plan ==\n")
+    appendOrError(rope.append)(
+      executedPlan.treeString(_, false, false, SQLConf.get.maxToStringFields))
+    rope.append("\n")
+
+    rope.toString
   }
 
   private def appendOrError(append: String => Unit)(f: (String => Unit) => Unit): Unit = {
@@ -210,6 +211,9 @@ class QueryExecution(
   }
 
   private def writePlans(append: String => Unit, maxFields: Int): Unit = {
+    def stringOrError[A](f: => A): String = {
+      try f.toString catch { case e: AnalysisException => e.toString }
+    }
     val (verbose, addSuffix) = (true, false)
 
     append("== Parsed Logical Plan ==\n")
@@ -234,15 +238,20 @@ class QueryExecution(
   }
 
   def stringWithStats: String = withRedaction {
+    val rope = new StringRope()
+    val maxFields = SQLConf.get.maxToStringFields
+
     // trigger to compute stats for logical plans
     optimizedPlan.stats
 
     // only show optimized logical plan and physical plan
-    s"""== Optimized Logical Plan ==
-        |${stringOrError(optimizedPlan.treeString(verbose = true, addSuffix = true))}
-        |== Physical Plan ==
-        |${stringOrError(executedPlan.treeString(verbose = true))}
-    """.stripMargin.trim
+    rope.append("== Optimized Logical Plan ==\n")
+    appendOrError(rope.append)(optimizedPlan.treeString(_, true, true, maxFields))
+    rope.append("\n== Physical Plan ==\n")
+    appendOrError(rope.append)(executedPlan.treeString(_, true, false, maxFields))
+    rope.append("\n")
+
+    rope.toString
   }
 
   /**

From d727959bc6602e6313ace85ca8bbd50da121ca1b Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 12:29:12 +0100
Subject: [PATCH 07/13] Added docs

---
 .../apache/spark/sql/catalyst/util/StringUtils.scala | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index 1a0cc4d80f6e..deeb3d4c75b6 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -88,10 +88,18 @@ object StringUtils {
     funcNames.toSeq
   }
 
+  /**
+   * Concatenation of sequence of strings to final string with cheap append method
+   * and one memory allocation for the final string.
+   */
   class StringRope {
     private var list = List.empty[String]
     private var length: Int = 0
 
+    /**
+     * Appends a string and accumulates its length to allocate a string buffer for all
+     * appended strings once in the toString method.
+     */
     def append(s: String): Unit = {
       if (s != null) {
         list = s :: list
@@ -99,6 +107,10 @@ object StringUtils {
       }
     }
 
+    /**
+     * The method allocates memory for all appended strings, writes them to the memory and
+     * returns concatenated string.
+     */
     override def toString: String = {
       val buffer = new StringBuffer(length)
       var reversed = list.reverse

From 544b80e0f9da6035635c2f2541930d8a5ebacbe9 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 12:42:20 +0100
Subject: [PATCH 08/13] Replacing List by ArrayBuffer

---
 .../spark/sql/catalyst/util/StringUtils.scala       | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index deeb3d4c75b6..f87200d1b438 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -19,6 +19,8 @@ package org.apache.spark.sql.catalyst.util
 
 import java.util.regex.{Pattern, PatternSyntaxException}
 
+import scala.collection.mutable.ArrayBuffer
+
 import org.apache.spark.sql.AnalysisException
 import org.apache.spark.unsafe.types.UTF8String
 
@@ -93,7 +95,7 @@ object StringUtils {
    * and one memory allocation for the final string.
    */
   class StringRope {
-    private var list = List.empty[String]
+    private val rope = new ArrayBuffer[String]
     private var length: Int = 0
 
     /**
@@ -102,7 +104,7 @@ object StringUtils {
      */
     def append(s: String): Unit = {
       if (s != null) {
-        list = s :: list
+        rope.append(s)
         length += s.length
       }
     }
@@ -113,13 +115,8 @@ object StringUtils {
      */
     override def toString: String = {
       val buffer = new StringBuffer(length)
-      var reversed = list.reverse
-
-      while (!reversed.isEmpty) {
-        buffer.append(reversed.head)
-        reversed = reversed.tail
-      }
 
+      rope.foreach(buffer.append)
       buffer.toString
     }
   }

From 059dcf493eba038a5df88a29ab7b6a837cdc07f0 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 13:03:45 +0100
Subject: [PATCH 09/13] Inline stringOrError

---
 .../apache/spark/sql/execution/QueryExecution.scala   | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index 8d1c7f60db6f..35153cd72ef7 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -211,16 +211,17 @@ class QueryExecution(
   }
 
   private def writePlans(append: String => Unit, maxFields: Int): Unit = {
-    def stringOrError[A](f: => A): String = {
-      try f.toString catch { case e: AnalysisException => e.toString }
-    }
     val (verbose, addSuffix) = (true, false)
 
     append("== Parsed Logical Plan ==\n")
     appendOrError(append)(logical.treeString(_, verbose, addSuffix, maxFields))
     append("\n== Analyzed Logical Plan ==\n")
-    val analyzedOutput = stringOrError(truncatedString(
-      analyzed.output.map(o => s"${o.name}: ${o.dataType.simpleString}"), ", ", maxFields))
+    val analyzedOutput = try {
+      truncatedString(
+        analyzed.output.map(o => s"${o.name}: ${o.dataType.simpleString}"), ", ", maxFields)
+    } catch {
+      case e: AnalysisException => e.toString
+    }
     append(analyzedOutput)
     append("\n")
     appendOrError(append)(analyzed.treeString(_, verbose, addSuffix, maxFields))

From fe3bbcf9e56e3436bd16541962a25b5c047da684 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 22:26:47 +0100
Subject: [PATCH 10/13] Move appendOrError to QueryPlan

---
 .../spark/sql/catalyst/plans/QueryPlan.scala  | 17 +++++++++++
 .../spark/sql/execution/QueryExecution.scala  | 28 ++++++-------------
 2 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
index 125181fb213f..8f5444ed8a5a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.sql.catalyst.plans
 
+import org.apache.spark.sql.AnalysisException
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.trees.{CurrentOrigin, TreeNode}
 import org.apache.spark.sql.internal.SQLConf
@@ -301,4 +302,20 @@ object QueryPlan extends PredicateHelper {
       Nil
     }
   }
+
+  /**
+   * Converts the query plan to string and appends it via provided function.
+   */
+  def append[T <: QueryPlan[T]](
+      plan: => QueryPlan[T],
+      append: String => Unit,
+      verbose: Boolean,
+      addSuffix: Boolean,
+      maxFields: Int = SQLConf.get.maxToStringFields): Unit = {
+    try {
+      plan.treeString(append, verbose, addSuffix, maxFields)
+    } catch {
+      case e: AnalysisException => append(e.toString)
+    }
+  }
 }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index 35153cd72ef7..273f8a7dda4e 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -27,6 +27,7 @@ import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.{AnalysisException, Row, SparkSession}
 import org.apache.spark.sql.catalyst.{InternalRow, QueryPlanningTracker}
 import org.apache.spark.sql.catalyst.analysis.UnsupportedOperationChecker
+import org.apache.spark.sql.catalyst.plans.QueryPlan
 import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, ReturnAnswer}
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.catalyst.util.DateTimeUtils
@@ -194,27 +195,16 @@ class QueryExecution(
 
   def simpleString: String = withRedaction {
     val rope = new StringRope()
-
     rope.append("== Physical Plan ==\n")
-    appendOrError(rope.append)(
-      executedPlan.treeString(_, false, false, SQLConf.get.maxToStringFields))
+    QueryPlan.append(executedPlan, rope.append, verbose = false, addSuffix = false)
     rope.append("\n")
-
     rope.toString
   }
 
-  private def appendOrError(append: String => Unit)(f: (String => Unit) => Unit): Unit = {
-    try f(append)
-    catch {
-      case e: AnalysisException => append(e.toString)
-    }
-  }
-
   private def writePlans(append: String => Unit, maxFields: Int): Unit = {
     val (verbose, addSuffix) = (true, false)
-
     append("== Parsed Logical Plan ==\n")
-    appendOrError(append)(logical.treeString(_, verbose, addSuffix, maxFields))
+    QueryPlan.append(logical, append, verbose, addSuffix, maxFields)
     append("\n== Analyzed Logical Plan ==\n")
     val analyzedOutput = try {
       truncatedString(
@@ -224,16 +214,15 @@ class QueryExecution(
     }
     append(analyzedOutput)
     append("\n")
-    appendOrError(append)(analyzed.treeString(_, verbose, addSuffix, maxFields))
+    QueryPlan.append(analyzed, append, verbose, addSuffix, maxFields)
     append("\n== Optimized Logical Plan ==\n")
-    appendOrError(append)(optimizedPlan.treeString(_, verbose, addSuffix, maxFields))
+    QueryPlan.append(optimizedPlan, append, verbose, addSuffix, maxFields)
     append("\n== Physical Plan ==\n")
-    appendOrError(append)(executedPlan.treeString(_, verbose, addSuffix, maxFields))
+    QueryPlan.append(executedPlan, append, verbose, addSuffix, maxFields)
   }
 
   override def toString: String = withRedaction {
     val rope = new StringRope()
-
     writePlans(rope.append, SQLConf.get.maxToStringFields)
     rope.toString
   }
@@ -247,11 +236,10 @@ class QueryExecution(
 
     // only show optimized logical plan and physical plan
     rope.append("== Optimized Logical Plan ==\n")
-    appendOrError(rope.append)(optimizedPlan.treeString(_, true, true, maxFields))
+    QueryPlan.append(optimizedPlan, rope.append, verbose = true, addSuffix = true, maxFields)
     rope.append("\n== Physical Plan ==\n")
-    appendOrError(rope.append)(executedPlan.treeString(_, true, false, maxFields))
+    QueryPlan.append(executedPlan, rope.append, verbose = true, addSuffix = false, maxFields)
     rope.append("\n")
-
     rope.toString
   }
 

From 707c80b97cd888b84afbfac4752e937ba529132f Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 22:33:50 +0100
Subject: [PATCH 11/13] Renaming StringRope to StringConcat

---
 .../spark/sql/catalyst/trees/TreeNode.scala   |  8 ++---
 .../spark/sql/catalyst/util/StringUtils.scala | 13 ++++----
 .../sql/catalyst/util/StringUtilsSuite.scala  |  4 +--
 .../spark/sql/execution/QueryExecution.scala  | 32 +++++++++----------
 .../spark/sql/execution/debug/package.scala   |  9 +++---
 5 files changed, 32 insertions(+), 34 deletions(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
index 202d1b527de6..570a019b2af7 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
@@ -35,7 +35,7 @@ import org.apache.spark.sql.catalyst.errors._
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.JoinType
 import org.apache.spark.sql.catalyst.plans.physical.{BroadcastMode, Partitioning}
-import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
+import org.apache.spark.sql.catalyst.util.StringUtils.StringConcat
 import org.apache.spark.sql.catalyst.util.truncatedString
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types._
@@ -480,10 +480,10 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       verbose: Boolean,
       addSuffix: Boolean = false,
       maxFields: Int = SQLConf.get.maxToStringFields): String = {
-    val rope = new StringRope()
+    val concat = new StringConcat()
 
-    treeString(rope.append, verbose, addSuffix, maxFields)
-    rope.toString
+    treeString(concat.append, verbose, addSuffix, maxFields)
+    concat.toString
   }
 
   def treeString(
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index f87200d1b438..50476001656e 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -94,8 +94,8 @@ object StringUtils {
    * Concatenation of sequence of strings to final string with cheap append method
    * and one memory allocation for the final string.
    */
-  class StringRope {
-    private val rope = new ArrayBuffer[String]
+  class StringConcat {
+    private val strings = new ArrayBuffer[String]
     private var length: Int = 0
 
     /**
@@ -104,7 +104,7 @@ object StringUtils {
      */
     def append(s: String): Unit = {
       if (s != null) {
-        rope.append(s)
+        strings.append(s)
         length += s.length
       }
     }
@@ -114,10 +114,9 @@ object StringUtils {
      * returns concatenated string.
      */
     override def toString: String = {
-      val buffer = new StringBuffer(length)
-
-      rope.foreach(buffer.append)
-      buffer.toString
+      val result = new StringBuffer(length)
+      strings.foreach(result.append)
+      result.toString
     }
   }
 }
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
index 42be9980e95a..411a64c125e5 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
@@ -46,10 +46,10 @@ class StringUtilsSuite extends SparkFunSuite {
 
   test("string rope") {
     def toRope(seq: String*): String = {
-      seq.foldLeft(new StringRope())((rope, s) => {rope.append(s); rope}).toString
+      seq.foldLeft(new StringConcat())((rope, s) => {rope.append(s); rope}).toString
     }
 
-    assert(new StringRope().toString == "")
+    assert(new StringConcat().toString == "")
     assert(toRope("") == "")
     assert(toRope(null) == "")
     assert(toRope("a") == "a")
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
index 273f8a7dda4e..7fccbf65d852 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala
@@ -31,7 +31,7 @@ import org.apache.spark.sql.catalyst.plans.QueryPlan
 import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, ReturnAnswer}
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.catalyst.util.DateTimeUtils
-import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
+import org.apache.spark.sql.catalyst.util.StringUtils.StringConcat
 import org.apache.spark.sql.catalyst.util.truncatedString
 import org.apache.spark.sql.execution.command.{DescribeTableCommand, ExecutedCommandExec, ShowTablesCommand}
 import org.apache.spark.sql.execution.exchange.{EnsureRequirements, ReuseExchange}
@@ -194,11 +194,11 @@ class QueryExecution(
   }
 
   def simpleString: String = withRedaction {
-    val rope = new StringRope()
-    rope.append("== Physical Plan ==\n")
-    QueryPlan.append(executedPlan, rope.append, verbose = false, addSuffix = false)
-    rope.append("\n")
-    rope.toString
+    val concat = new StringConcat()
+    concat.append("== Physical Plan ==\n")
+    QueryPlan.append(executedPlan, concat.append, verbose = false, addSuffix = false)
+    concat.append("\n")
+    concat.toString
   }
 
   private def writePlans(append: String => Unit, maxFields: Int): Unit = {
@@ -222,25 +222,25 @@ class QueryExecution(
   }
 
   override def toString: String = withRedaction {
-    val rope = new StringRope()
-    writePlans(rope.append, SQLConf.get.maxToStringFields)
-    rope.toString
+    val concat = new StringConcat()
+    writePlans(concat.append, SQLConf.get.maxToStringFields)
+    concat.toString
   }
 
   def stringWithStats: String = withRedaction {
-    val rope = new StringRope()
+    val concat = new StringConcat()
     val maxFields = SQLConf.get.maxToStringFields
 
     // trigger to compute stats for logical plans
     optimizedPlan.stats
 
     // only show optimized logical plan and physical plan
-    rope.append("== Optimized Logical Plan ==\n")
-    QueryPlan.append(optimizedPlan, rope.append, verbose = true, addSuffix = true, maxFields)
-    rope.append("\n== Physical Plan ==\n")
-    QueryPlan.append(executedPlan, rope.append, verbose = true, addSuffix = false, maxFields)
-    rope.append("\n")
-    rope.toString
+    concat.append("== Optimized Logical Plan ==\n")
+    QueryPlan.append(optimizedPlan, concat.append, verbose = true, addSuffix = true, maxFields)
+    concat.append("\n== Physical Plan ==\n")
+    QueryPlan.append(executedPlan, concat.append, verbose = true, addSuffix = false, maxFields)
+    concat.append("\n")
+    concat.toString
   }
 
   /**
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
index d52d889318cf..53b74c7c8559 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala
@@ -29,7 +29,7 @@ import org.apache.spark.sql.catalyst.expressions.Attribute
 import org.apache.spark.sql.catalyst.expressions.codegen.{CodeFormatter, CodegenContext, ExprCode}
 import org.apache.spark.sql.catalyst.plans.physical.Partitioning
 import org.apache.spark.sql.catalyst.trees.TreeNodeRef
-import org.apache.spark.sql.catalyst.util.StringUtils.StringRope
+import org.apache.spark.sql.catalyst.util.StringUtils.StringConcat
 import org.apache.spark.sql.execution.streaming.{StreamExecution, StreamingQueryWrapper}
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.streaming.StreamingQuery
@@ -71,10 +71,9 @@ package object debug {
    * @return single String containing all WholeStageCodegen subtrees and corresponding codegen
    */
   def codegenString(plan: SparkPlan): String = {
-    val rope = new StringRope()
-
-    writeCodegen(rope.append, plan)
-    rope.toString
+    val concat = new StringConcat()
+    writeCodegen(concat.append, plan)
+    concat.toString
   }
 
   def writeCodegen(append: String => Unit, plan: SparkPlan): Unit = {

From 074e9b87716cb3c59ef3a4cb59ba39e070886c55 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Sun, 30 Dec 2018 22:36:57 +0100
Subject: [PATCH 12/13] rope -> concat in tests

---
 .../sql/catalyst/util/StringUtilsSuite.scala     | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
index 411a64c125e5..616ec12032db 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/StringUtilsSuite.scala
@@ -44,16 +44,16 @@ class StringUtilsSuite extends SparkFunSuite {
     assert(filterPattern(names, " d* ") === Nil)
   }
 
-  test("string rope") {
-    def toRope(seq: String*): String = {
-      seq.foldLeft(new StringConcat())((rope, s) => {rope.append(s); rope}).toString
+  test("string concatenation") {
+    def concat(seq: String*): String = {
+      seq.foldLeft(new StringConcat())((acc, s) => {acc.append(s); acc}).toString
     }
 
     assert(new StringConcat().toString == "")
-    assert(toRope("") == "")
-    assert(toRope(null) == "")
-    assert(toRope("a") == "a")
-    assert(toRope("1", "2") == "12")
-    assert(toRope("abc", "\n", "123") == "abc\n123")
+    assert(concat("") == "")
+    assert(concat(null) == "")
+    assert(concat("a") == "a")
+    assert(concat("1", "2") == "12")
+    assert(concat("abc", "\n", "123") == "abc\n123")
   }
 }

From 29b62bf4365bfadd77e4c44b718124b6764471e0 Mon Sep 17 00:00:00 2001
From: Maxim Gekk <maxim.gekk@databricks.com>
Date: Mon, 31 Dec 2018 10:31:47 +0100
Subject: [PATCH 13/13] StringBuffer -> StringBuilder

---
 .../scala/org/apache/spark/sql/catalyst/util/StringUtils.scala  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
index 50476001656e..643b83b1741a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala
@@ -114,7 +114,7 @@ object StringUtils {
      * returns concatenated string.
      */
     override def toString: String = {
-      val result = new StringBuffer(length)
+      val result = new java.lang.StringBuilder(length)
       strings.foreach(result.append)
       result.toString
     }