private static String extractColumnName(ExprNodeDesc expr) throws SemanticException {
    if (expr instanceof ExprNodeColumnDesc) {
      ExprNodeColumnDesc exprNodeColumnDesc = (ExprNodeColumnDesc) expr;
      return exprNodeColumnDesc.getColumn();

    }
    if (expr instanceof ExprNodeConstantDesc) {
      ExprNodeConstantDesc exprNodeConstantDesc = (ExprNodeConstantDesc) expr;
      return exprNodeConstantDesc.getFoldedFromCol();
    }
    throw new SemanticException("unexpected mapping expression!");
  }
Fixes a parallel edge going into a mapjoin by introducing a concentrator RS.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ParallelEdgeFixer.java