Flush handlers with meta: flush_handlers before the task that requires it:
- name: change postgre pg_hba.conf
  template: src=pg_hba.conf.j2 dest=/etc/postgresql/9.4/main/pg_hba.conf 
  notify: restart postgresql service
  tags: pg_hba
- meta: flush_handlers
- name: set password for postgres
  postgresql_user:
    db: postgres
    user: postgres
    password: postgres