public static ASTNode getASTNode(ColumnInfo cInfo, RowResolver rr) throws SemanticException {
    for (Map.Entry<String, ASTNode> entry : rr.getExpressionMap().entrySet()) {
      ASTNode expr = entry.getValue();
      if (rr.getExpression(expr).equals(cInfo)) {
        return expr;
      }
    }
    return null;
  }
For each column on the input RR, construct a StructField for it
OI is constructed using the list of input column names and
their corresponding OIs.

@param rr
@return
ql/src/java/org/apache/hadoop/hive/ql/parse/PTFTranslator.java