mysqlではconnectionごとにthreadを作成します

 mysqld_main     ，     code           
mysqld_socket_acceptor->connection_event_loop();
  void connection_event_loop() {
    Connection_handler_manager *mgr =
        Connection_handler_manager::get_instance();
    while (!connection_events_loop_aborted()) {
      Channel_info *channel_info = m_listener->listen_for_connection_event();
      if (channel_info != NULL) mgr->process_new_connection(channel_info);
    }
  }

      listen_for_connection_event，      process_new_connection   
void Connection_handler_manager::process_new_connection(
    Channel_info *channel_info) {
  if (connection_events_loop_aborted() || !check_and_incr_conn_count()) {
    channel_info->send_error_and_close_channel(ER_CON_COUNT_ERROR, 0, true);
    delete channel_info;
    return;
  }
#    add_connection    connection，       ，     connection     thread，     connection     thread，     connection     thread   ，          
  if (m_connection_handler->add_connection(channel_info)) {
    inc_aborted_connects();
    delete channel_info;
  }
}

add_connection     ：

bool Per_thread_connection_handler::add_connection(Channel_info *channel_info) {
  int error = 0;
  my_thread_handle id;

  DBUG_ENTER("Per_thread_connection_handler::add_connection");

  // Simulate thread creation for test case before we check thread cache
  DBUG_EXECUTE_IF("fail_thread_create", error = 1; goto handle_error;);

  if (!check_idle_thread_and_enqueue_connection(channel_info))
    DBUG_RETURN(false);

  /*
    There are no idle threads avaliable to take up the new
    connection. Create a new thread to handle the connection
  */
  channel_info->set_prior_thr_create_utime();
#  mysql_thread_create    thread，   mysql_thread_create   pthread_create   ，         
#thread   client connetion，          handle_connection
  error =
      mysql_thread_create(key_thread_one_connection, &id, &connection_attrib,
                          handle_connection, (void *)channel_info);

}


static void *handle_connection(void *arg) {
  

  for (;;) {
    THD *thd = init_new_thd(channel_info);
    thd_manager->add_thd(thd);

    if (thd_prepare_connection(thd))
      handler_manager->inc_aborted_connects();
    else {
      while (thd_connection_alive(thd)) {
#         do_command    client       
        if (do_command(thd)) break;
      }
      end_connection(thd);
    }
  }

}