Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/4940#discussion_r26013756
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateProjection.scala
 ---
    @@ -84,9 +84,15 @@ object GenerateProjection extends 
CodeGenerator[Seq[Expression], Projection] {
             val elementName = newTermName(s"c$i")
             val iLit = ru.Literal(Constant(i))
     
    -        q"if(i == $ordinal) { if(isNullAt($i)) return null else return 
$elementName }"
    +        q"if(i == $ordinal) { return $elementName }"
           }
    -      q"override def apply(i: Int): Any = { ..$cases; $accessorFailure }"
    +      q"""
    +        override def apply(i: Int): Any = {
    +          if (i < 0 || i >= this.length) $accessorFailure
    --- End diff --
    
    It is used to check if the parameter `i` fails in legal range. `if(i == 
$ordinal)` is used to choose the corresponding element to return. Checking the 
legal range and giving `accessorFailure` in advance can avoid further running 
of `$cases`. (when row size grows, that will be many `if` checks.)


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