Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/19964#discussion_r156685033
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala
 ---
    @@ -289,53 +289,56 @@ case class Elt(children: Seq[Expression])
         val index = indexExpr.genCode(ctx)
         val strings = stringExprs.map(_.genCode(ctx))
         val indexVal = ctx.freshName("index")
    +    val indexMatched = ctx.freshName("eltIndexMatched")
    +
         val stringVal = ctx.freshName("stringVal")
    +    ctx.addMutableState(ctx.javaType(dataType), stringVal)
    +
         val assignStringValue = strings.zipWithIndex.map { case (eval, index) 
=>
           s"""
    -        case ${index + 1}:
    -          ${eval.code}
    -          $stringVal = ${eval.isNull} ? null : ${eval.value};
    -          break;
    -      """
    +         |if ($indexVal == ${index + 1}) {
    +         |  ${eval.code}
    +         |  $stringVal = ${eval.isNull} ? null : ${eval.value};
    +         |  $indexMatched = true;
    +         |  continue;
    +         |}
    +      """.stripMargin
         }
     
    -    val cases = ctx.buildCodeBlocks(assignStringValue)
    -    val codes = if (cases.length == 1) {
    -      s"""
    -        UTF8String $stringVal = null;
    -        switch ($indexVal) {
    -          ${cases.head}
    -        }
    -       """
    -    } else {
    -      var prevFunc = "null"
    -      for (c <- cases.reverse) {
    -        val funcName = ctx.freshName("eltFunc")
    -        val funcBody = s"""
    -         private UTF8String $funcName(InternalRow ${ctx.INPUT_ROW}, int 
$indexVal) {
    -           UTF8String $stringVal = null;
    -           switch ($indexVal) {
    -             $c
    -             default:
    -               return $prevFunc;
    -           }
    -           return $stringVal;
    -         }
    -        """
    -        val fullFuncName = ctx.addNewFunction(funcName, funcBody)
    -        prevFunc = s"$fullFuncName(${ctx.INPUT_ROW}, $indexVal)"
    -      }
    -      s"UTF8String $stringVal = $prevFunc;"
    -    }
    +    val codes = ctx.splitExpressionsWithCurrentInputs(
    +      expressions = assignStringValue,
    +      funcName = "eltFunc",
    +      extraArguments = ("int", indexVal) :: Nil,
    +      returnType = ctx.JAVA_BOOLEAN,
    +      makeSplitFunction = body =>
    +        s"""
    +           |${ctx.JAVA_BOOLEAN} $indexMatched = false;
    +           |do {
    +           |  $body
    +           |} while (false);
    +           |return $indexMatched;
    +         """.stripMargin,
    +      foldFunctions = _.map { funcCall =>
    +        s"""
    +           |$indexMatched = $funcCall;
    +           |if ($indexMatched) {
    +           |  continue;
    +           |}
    +         """.stripMargin
    +      }.mkString)
     
         ev.copy(
           s"""
    -      ${index.code}
    -      final int $indexVal = ${index.value};
    -      $codes
    -      UTF8String ${ev.value} = $stringVal;
    -      final boolean ${ev.isNull} = ${ev.value} == null;
    -    """)
    +         |${index.code}
    +         |final int $indexVal = ${index.value};
    +         |${ctx.JAVA_BOOLEAN} $indexMatched = false;
    +         |$stringVal = ${ctx.defaultValue(dataType)};
    --- End diff --
    
    +1, since at the end we do `final boolean ${ev.isNull} = ${ev.value} == 
null;`.


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