max_connections 是指MySQL服務器的最大連接數。即所有用戶最大連接數的和。
max_user_connections 是指MySQL中單個用戶的最大連接數。
這里說明當前用戶的連接數大于了單個用戶的最大連接數，需要擴大連接數：

mysql> show variables like '%connect%';
+-----------------------------------------------+-----------------+
| Variable_name                                 | Value           |
+-----------------------------------------------+-----------------+
| character_set_connection                      | utf8mb3         |
| collation_connection                          | utf8_general_ci |
| connect_timeout                               | 10              |
| disconnect_on_expired_password                | ON              |
| init_connect                                  |                 |
| max_connect_errors                            | 100             |
| max_connections                               | 4000            |
| max_user_connections                          | 1500            |
| mysqlx_connect_timeout                        | 30              |
| mysqlx_max_connections                        | 100             |
| performance_schema_session_connect_attrs_size | 512             |
+-----------------------------------------------+-----------------+
11 rows in set (0.00 sec)mysql> set global max_user_connections=4000;
Query OK, 0 rows affected (0.00 sec)mysql> show variables like '%connect%';
+-----------------------------------------------+-----------------+
| Variable_name                                 | Value           |
+-----------------------------------------------+-----------------+
| character_set_connection                      | utf8mb3         |
| collation_connection                          | utf8_general_ci |
| connect_timeout                               | 10              |
| disconnect_on_expired_password                | ON              |
| init_connect                                  |                 |
| max_connect_errors                            | 100             |
| max_connections                               | 4000            |
| max_user_connections                          | 4000            |
| mysqlx_connect_timeout                        | 30              |
| mysqlx_max_connections                        | 100             |
| performance_schema_session_connect_attrs_size | 512             |
+-----------------------------------------------+-----------------+
11 rows in set (0.00 sec)