mysql 执行命令的过程

当有客户端发送命令道mysqld的时候，会在下面函数中接受命令
mysql-server\sql\conn_handler\connection_acceptor.h中的
  /**
    Connection acceptor loop to accept connections from clients.
  */
  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();
				#建立新的连接
      if (channel_info != NULL) mgr->process_new_connection(channel_info);
    }
  }
mysql-server\sql\conn_handler\connection_handler_manager.cc
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;
  }
#通过add_connection函数负责添加连接
  if (m_connection_handler->add_connection(channel_info)) {
    inc_aborted_connects();
    delete channel_info;
  }
}
这里注意  virtual bool add_connection(Channel_info *channel_info) = 0; 是一个虚函数

这个虚函数有两个实现
mysql-server\sql\conn_handler\connection_handler_impl.h
  Per_thread_connection_handler() {}
  virtual ~Per_thread_connection_handler() {}

 protected:
  virtual bool add_connection(Channel_info *channel_info);

  virtual uint get_max_threads() const;
};

/**
  This class represents the connection handling functionality
  of all connections being handled in a single worker thread.
*/
class One_thread_connection_handler : public Connection_handler {
  One_thread_connection_handler(const One_thread_connection_handler &);
  One_thread_connection_handler &operator=(
      const One_thread_connection_handler &);

 public:
  One_thread_connection_handler() {}
  virtual ~One_thread_connection_handler() {}

 protected:
  virtual bool add_connection(Channel_info *channel_info);

  virtual uint get_max_threads() const { return 1; }
};

这里以Per_thread_connection_handler中的实现为例
mysql-server\sql\conn_handler\connection_handler_per_thread.cc
bool Per_thread_connection_handler::add_connection(Channel_info *channel_info) {
  int error = 0;
  my_thread_handle id;

  /*
    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();
#进行线程的创建，这里的回调函数是handle_connection, 
  error =
      mysql_thread_create(key_thread_one_connection, &id, &connection_attrib,
                          handle_connection, (void *)channel_info);

  Global_THD_manager::get_instance()->inc_thread_created();
  DBUG_PRINT("info", ("Thread created"));
  DBUG_RETURN(false);
}
handle_connection同样在当前文件中实现
static void *handle_connection(void *arg) {
  Global_THD_manager *thd_manager = Global_THD_manager::get_instance();

    if (thd_prepare_connection(thd))
      handler_manager->inc_aborted_connects();
    else {
      while (thd_connection_alive(thd)) {
#调用do_command来处理客户端发送过来的sql语句
        if (do_command(thd)) break;
      }
      end_connection(thd);
    }

mysql-server\sql\sql_parse.cc
bool do_command(THD *thd) {
#调用dispatch_command 来进行命令的分发
  return_value = dispatch_command(thd, &com_data, command);
  thd->get_protocol_classic()->get_output_packet()->shrink(
      thd->variables.net_buffer_length);
}
mysql-server\sql\sql_parse.cc
bool dispatch_command(THD *thd, const COM_DATA *com_data,
                      enum enum_server_command command) {
  case COM_QUERY: {
      DBUG_ASSERT(thd->m_digest == NULL);
      thd->m_digest = &thd->m_digest_state;
      thd->m_digest->reset(thd->m_token_array, max_digest_length);



      Parser_state parser_state;
      if (parser_state.init(thd, thd->query().str, thd->query().length)) break;
	#进行命令的解析
      mysql_parse(thd, &parser_state);


}

void mysql_parse(THD *thd, Parser_state *parser_state) {
#对解析完的命令进行执行
error = mysql_execute_command(thd, true);

int mysql_execute_command(THD *thd, bool first_level) {

case SQLCOM_SELECT:
     DBUG_ASSERT(lex->m_sql_cmd != nullptr);
#下面就是对个个指令的执行
      res = lex->m_sql_cmd->execute(thd);
      break;
}
}