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

    https://github.com/apache/spark/pull/14277#discussion_r71648156
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala
 ---
    @@ -204,6 +204,29 @@ case class Elt(children: Seq[Expression])
           }
         }
       }
    +
    +  override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): 
ExprCode = {
    +    val index = children.head.map(_.genCode(ctx))(0)
    +    val strings = children.tail.map(_.genCode(ctx))
    +    val stringValues = strings.map { eval =>
    +      s"${eval.isNull} ? null : ${eval.value}"
    +    }.mkString(", ")
    +    val indexVal = ctx.freshName("index")
    +    val stringArray = ctx.freshName("strings");
    +
    +    ev.copy(index.code + "\n" + strings.map(_.code).mkString("\n") + s"""
    +      int $indexVal = ${index.value} - 1;
    +      UTF8String[] $stringArray = {$stringValues};
    --- End diff --
    
    no, expressions can always codegen, by default we just pass the expression 
reference into generated java code and call its `eval` method. So it's only 
about performance for the expression.


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