A Comprehensive Guide: How to Find and Optimize Slow Queries in MySQL

Everyone knows that website performance, or simply put, the speed at which a website renders pages, is the cornerstone for promotion, growth in popularity and of course maintaining a high ranking in search engines. And it depends not only and not so much on the performance of your server. After all, no one likes it when, trying to open a site, we have to wait more than one second :)
And one of the most important factors that can affect the speed of the site is the speed of the site database.
Slow queries against your MySQL database can slow down your website's responsiveness, frustrating users and impacting your business. In this article, we'll look at practical steps you can take to identify and optimize slow queries in MySQL to keep your database running smoothly and efficiently. Let's dive in!

Step 1: Enabling the MySQL Slow Query Log
To begin the process of identifying slow queries, you need to enable the MySQL Slow Query Log. This feature records queries that take longer than a specific threshold to execute, allowing you to pinpoint the problematic queries. You can enable the Slow Query Log by modifying the MySQL configuration file, commonly known as my.cnf.

For example, open the my.cnf file and add the following lines:

slow_query_log = 1
slow_query_log_file = /var/log/mysql/slow-query.log
long_query_time = 2

In this example, we've enabled the slow query log, specified the log file's location, and set the threshold to 2 seconds. Feel free to adjust the threshold according to your specific requirements.

Step 2: Analyzing the Slow Query Log
With the Slow Query Log enabled and configured, you can now analyze its contents to identify the slow queries. There are various tools available to help you examine the log file and extract the necessary information. One commonly used method is to utilize the MySQL command-line tool.

Open your terminal and run the following command:

mysql -u your_username -p < /var/log/mysql/slow-query.log

This command will display the slow queries recorded in the log file. Take note of the query execution time, query type, and the specific tables being accessed.

Step 3: Explaining and Optimizing Slow Queries
To optimize slow queries, you need to understand how MySQL processes them. This is where the EXPLAIN statement comes into play. By running EXPLAIN on a slow query, you can gain valuable insights into the query's execution plan and identify potential bottlenecks.

Let's consider an example. Suppose you have a slow query that retrieves information from a table named "customers." Run the following query:

EXPLAIN SELECT * FROM customers WHERE age > 30;

The EXPLAIN statement will provide detailed information about how MySQL executes the query, including the type of access used, indexes utilized, and the order in which the tables are accessed. By analyzing this information, you can make informed decisions about optimizing your queries.

Optimization techniques may include adding appropriate indexes, rewriting queries to be more efficient, or restructuring your database schema if necessary. Remember to retest your queries after implementing optimizations to ensure improved performance.

Step 4: Leveraging MySQL Performance Schema
MySQL Performance Schema is a powerful tool that provides detailed information about database performance, including query execution, resource utilization, and other essential metrics. You can leverage the Performance Schema to identify slow queries, analyze query execution plans, and monitor resource-intensive queries.

For example, you can use the following query to identify the top slowest queries:

SELECT * FROM performance_schema.events_statements_summary_by_digest ORDER BY sum_timer_wait DESC LIMIT 10;

This query retrieves the top 10 slowest queries based on the sum of the timer waits. Analyzing this data will help you pinpoint critical areas for optimization.

Step 5: Implementing Query Cache
MySQL's query cache can significantly improve the performance of frequently executed queries by storing the result sets in memory. Enabling the query cache reduces the load on the database, resulting in faster response times for repeated queries.

To enable the query cache, open your my.cnf file and add the following lines:

query_cache_type = 1
query_cache_size = 64M

In this example, we've set the query cache type to 1 (enable) and allocated 64 megabytes of memory for the cache. Adjust the cache size according to your server's available memory.

These are five easy steps to start looking for bottlenecks on your site and of course, in this article we have covered only the most basic steps, but following them will allow you to find most of the problem requests due to which your site freezes or does not respond at all.

It would not be superfluous to also recall that if you do not understand the essence of the issue at all, then it is better to entrust the work of optimizing the database to a specialist, since inept hands can completely break the database. Therefore, all actions must be performed consciously, with knowledge of the matter.