1. If the connection fails, check the service status, port opening, account permissions and the upper limit of the number of connections; 2. Common error codes such as 1045, 2003, 1206, and 1040 need to be targeted for network, permissions, configuration and connection pool optimization; 3. Check the index, slow log, system resources and transaction submission method first; 4. Crash recovery requires regular backup and test logical and physical backups, enable binlog and monitor master-slave synchronization. The above steps can quickly locate MySQL operation problems by sequentially.

It is common for MySQL errors to occur, especially when a project encounters sudden problems after it is launched, it is very important to quickly locate and resolve errors. This article will not talk about "how to install MySQL", but will directly enter the topic: the recognition and handling methods of common MySQL server errors to help you avoid detours in actual use.

1. Can't connect to MySQL? Check these basic items

Connection failure is one of the most common problems. Users often report that "cannot connect to the database", but the reasons may be diverse:

• Is MySQL started?
  The most basic issues are also the easiest to be overlooked. You can use systemctl status mysql or service mysql status to check the service status.

• Is the port open?
  The default port is 3306. If you are using a cloud server, you need to confirm whether the security group or firewall releases the port.

  

• Is the account password correct?
  The login failure may be due to a wrong password or the user permission configuration may be due to the wrong user permissions. For example, users can only log in from local, while you try to access from remote.

• Is max_connections reaching the upper limit?
  If there are too many concurrent connections, this limit may be triggered. You can use to view the current connection number:

   SHOW STATUS LIKE 'Threads_connected';

It is recommended to give priority to these points, and many times the problems lie in these basic configurations.

2. Interpretation of common error codes and response strategies

MySQL error messages usually contain an error code and description. Understanding several common error codes can save a lot of time:

• Error 1045: Access denied for user
  Indicates that the user name or password is wrong, or that the user does not have permission to connect from the current host. You can check whether Host field in the mysql.user table matches.

• Error 2003: Can't connect to MySQL server on 'xxx'
  Network-level problems may be IP unreachable, port not listened to, or firewall intercept. ping first and then telnet to see if the port is connected.

• Error 1206: The maximum number of active extensions has been reached
  It means that the current number of connections exceeds the system limit. You can adjust it by modifying the max_connections configuration, but you should also pay attention to whether the server resources are sufficient.

• Error 1040: Too many connections
  Also, there are too many connections, consider optimizing the program connection pool settings to avoid frequent establishment of new connections in a short period of time.

When such errors occur, it is recommended to further analyze them in combination with log files (such as /var/log/mysql/error.log ).

3. Slow query? Don't rush to adjust SQL, look at these indicators first

Many users suspect that SQL is not well written when they encounter slow queries. In fact, it is sometimes a configuration or index problem:

• Is there a suitable index?
  Use EXPLAIN to analyze the query plan and see if there are full table scans. Especially for large tables, the lack of suitable indexes will seriously affect performance.

• Is slow query log enabled?
  When enabled, SQL whose execution time exceeds the specified threshold can be recorded, which helps to identify potential bottlenecks. Related parameters include:

  • slow_query_log
  • long_query_time

• Is the server resources tight?
  CPU, memory, and disk IO may all affect database performance. You can use top , iostat and other commands to view the system load.

• Is unnecessary automatic submission enabled?
  In batch write operations, turning off automatic commit ( autocommit=0 ) and manually commit transactions can significantly improve efficiency.

Optimizing queries is not just about changing sentences, but also about the overall environment.

4. Database crash? Backup and recovery mechanisms cannot be missing

Although it does not happen every day, once the database crashes, whether or not there is a perfect backup mechanism is life or death:

• Regularly do logical backups (mysqldump) and physical backups (xtrabackup)
  Choose a backup method according to business needs. MySQLdump is available for small data volume, and xtrabackup is recommended for large data volume.

• Enable binary logging (binlog)
  binlog can be used to recover accidentally deleted data or perform point-in-time recovery. It is recommended to keep log files for a period of time.

• Test recovery process
  Many people only backup and do not test, but only find that they cannot recover when something really happens. It is recommended to practice the recovery process regularly to ensure backups are available.

• Monitor master-slave replication status (if it is a high-availability architecture)
  The master-slave is out of sync and the data will be risked. You can view the replication status through SHOW SLAVE STATUS\G .

The stable operation of the database is inseparable from good operation and maintenance habits, and preparation in advance is much more important than remediation afterwards.

Basically that's it. These problems occur frequently in daily use and are not particularly complicated to deal with, but they are prone to repeated errors due to inadequate details. If you master these ideas, you can handle them yourself most of the time.

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