@Override
  public List<Task<?>> getAllRootTasks() {
    if (!rootTasksResolved) {
      LinkedHashMap<CTEClause, LinkedHashSet<CTEClause>> realDependencies = listRealDependencies();
      linkRealDependencies(realDependencies);
      rootTasks = toRealRootTasks(realDependencies);
      rootTasksResolved = true;
    }
    return rootTasks;
  }
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