public static SessionState start(SessionState startSs) {
    start(startSs, false, null);

    // Register the session state in the centralized set
    sessionStates.add(startSs);

    return startSs;
  }
set current session to existing session object if a thread is running
multiple sessions - it must call this method with the new session object
when switching from one session to another.
ql/src/java/org/apache/hadoop/hive/ql/session/SessionState.java