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

    https://github.com/apache/flink/pull/311#discussion_r23070484
  
    --- Diff: 
flink-compiler/src/main/java/org/apache/flink/compiler/dag/BinaryUnionNode.java 
---
    @@ -266,4 +268,44 @@ public void computeOutputEstimates(DataStatistics 
statistics) {
                this.estimatedOutputSize = in1.estimatedOutputSize > 0 && 
in2.estimatedOutputSize > 0 ?
                        in1.estimatedOutputSize + in2.estimatedOutputSize : -1;
        }
    +
    +   public static class UnionSemanticProperties implements 
SemanticProperties {
    +
    +           @Override
    +           public FieldSet getTargetFields(int input, int sourceField) {
    +                   if (input != 0 && input != 1) {
    +                           throw new IndexOutOfBoundsException();
    --- End diff --
    
    That's an internal exception. If something fails here, there is something 
wrong with the optimizer. Nothing a user can solve. Could make that clear 
though...


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