mysql slow query enabled

MySQL is a very popular relational database management system that is widely used, especially in the field of web development. However, if slow queries occur in MySQL, it will seriously affect the performance of the database.

In order to solve this problem, we need to enable MySQL's slow query function. This article will introduce how to enable slow query in MySQL and configure the corresponding parameters to optimize query performance.

1. What is slow query?

In MySQL, if the time to execute a SQL query statement exceeds a certain threshold, then the query is called a slow query. Typically, a slow query is defined as a query that takes more than one second, but this threshold can be adjusted on a case-by-case basis.

Slow queries are usually caused by the following reasons:

1. The SQL query statement itself is not optimized enough.
2. The index in the database is not complete enough.
3. The physical structure design of the database is unreasonable.

Slow queries will cause the CPU usage and memory usage of the database server to soar, seriously affecting the performance of MySQL. Therefore, we need to locate the cause of slow query as soon as possible and optimize it.

2. How to enable slow query?

In MySQL, it is very simple to enable the slow query function. We only need to add the following parameters to the MySQL configuration file:

log-slow-queries = /var/log/mysql/mysql-slow.log
long_query_time = 1

Among them, the log-slow-queries parameter is used to specify slow queries. The path and file name of the log file. The long_query_time parameter is used to specify the query time threshold in seconds. In this example, queries that take longer than 1 second are written to the slow query log file.

After adding these two parameters, we need to restart the MySQL service in order to apply the new configuration. In the CentOS system, we can use the following command to restart the MySQL service:

systemctl restart mysqld

Of course, this command may also vary depending on the system, please adjust it according to the specific situation.

3. How to analyze slow query logs?

After turning on the slow query log, we need to analyze the slow query log regularly in order to discover and solve the problem of slow query. We can use the mysqldumpslow tool that comes with MySQL to analyze slow query logs. This tool supports multiple sorting methods and can easily help us find the cause of slow queries.

The following are several commonly used commands:

# 按查询次数从大到小排序
mysqldumpslow -s c /var/log/mysql/mysql-slow.log

# 按查询时间从大到小排序
mysqldumpslow -s t /var/log/mysql/mysql-slow.log

# 按查询锁定的行数从大到小排序
mysqldumpslow -s l /var/log/mysql/mysql-slow.log

Before using these commands, we need to ensure that we have permission to access the slow query log file. Normally, the slow query log file is located under the /var/log/mysql/mysql-slow.log path.

Analyzing slow query logs is not an easy task and requires certain experience and skills. Usually, we analyze slow query logs based on multiple dimensions such as the execution time of the query, the number of queries, the number of locked rows in the query, etc., in order to find the direction for optimization.

4. How to optimize slow queries?

After analyzing the slow query log, we need to propose an optimization plan based on the analysis results. The following are several common optimization solutions:

1. Optimize SQL query statements: Modify SQL query statements, use indexes as much as possible, and avoid using query methods such as full table scans.
2. Optimize indexes: Add or modify indexes to tables in the database to complete query operations faster.
3. Optimize the physical structure: Adjust the physical structure of the database, including table partitioning, partitioning and other operations, in order to better manage the database.
4. Optimize caching: Use caching technology to reduce the number of database queries as much as possible, thereby increasing the data query speed.

In short, optimizing slow queries is a very complex process, which requires us to fully consider the physical, logical structure, query statements and other factors of the database. During the optimization process, it is necessary to minimize the interference to the database system while maintaining system stability.

5. Summary

It is very simple to enable the slow query function in MySQL, but analyzing the slow query log and optimizing the slow query is a very complicated task. Through the introduction of this article, I hope readers can master the method of enabling the slow query function in MySQL, master the skills of using the mysqldumpslow tool, and master the optimization methods for slow queries. In actual development, we need to reasonably optimize the database system based on factors such as business needs and user visits to better meet user needs.

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