private void assertCombineInputFormat(Tree numerator, String message) throws SemanticException {
    String inputFormat = conf.getVar(HiveConf.ConfVars.HIVE_EXECUTION_ENGINE).equals("tez") ?
        HiveConf.getVar(conf, HiveConf.ConfVars.HIVE_TEZ_INPUT_FORMAT):
        HiveConf.getVar(conf, HiveConf.ConfVars.HIVE_INPUT_FORMAT);
    if (!inputFormat.equals(CombineHiveInputFormat.class.getName())) {
      throw new SemanticException(generateErrorMessage((ASTNode) numerator,
          message + " sampling is not supported in " + inputFormat));
    }
  }
Goes though the tabref tree and finds the alias for the table. Once found,
it records the table name-> alias association in aliasToTabs. It also makes
an association from the alias to the table AST in parse info.

@return the alias of the table
ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java