求使用 flask-sqlalchemy 的正确姿势，最近总报 Too many connections 错误， SQLALCHEMY_POOL_SIZE=5

如题，已经设置了 SQLALCHEMY_POOL_SIZE=5 ，依然无法解决。

mysql> show global status like '%connect%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| Aborted_connects | 8 |
| Connections | 374 |
| Max_used_connections | 21 |
| Ssl_client_connects | 0 |
| Ssl_connect_renegotiates | 0 |
| Ssl_finished_connects | 0 |
| Threads_connected | 5 |
+--------------------------+-------+