MySQL Optimization


This article contains descriptions of some known issues and recipes that might help to get the best application performance.

How To Avoid Performance Issues When MySQL Data is Stored on HDD

It is recommended to use SSD to store the data in the MySQL 8.X database. However, if you do need to use the HDD, follow the steps described in the optimizing InnoDB Disk I/O article to avoid performance issues and set the following configuration parameters in the /etc/my.cnf file:

innodb_file_per_table = 0
wait_timeout = 28800

How To Avoid Performance Issues with MySQL optimizer

To minimize the risk of long compilations of SQL queries (which sometimes may take hours or even days; for details, see MySQL documentation), set optimizer_search_depth to 0:

optimizer_search_depth = 0

Usage of The utf8mb4 Character Set (The Full 4-Byte UTF-8 Unicode Encoding) in MySQL

To store supplementary characters (such as 4-byte emojis), configure the options file to use the utf8mb4 character set. Put the following configuration settings in your options file /etc/my.cnf:

default-character-set = utf8mb4

default-character-set = utf8mb4

character-set-server = utf8mb4
collation-server = utf8mb4_unicode_ci

MySQL also can load default options from another file (not /etc/my.cnf). In such cases, you have to put the configuration settings in this file. To find out which configuration files your MySQL server uses, run the following command:

$ mysqld --help --verbose 2> /dev/null | grep -A1 'Default options'

You will get the output with MySQL config files names similar to this:

Default options are read from the following files in the given order:
/etc/my.cnf /etc/mysql/my.cnf /usr/local/etc/my.cnf ~/.my.cnf


You also can set up the character set and the collation on the other levels as well.

You can find more information on MySQL configuration in the Unicode Support and InnoDB File-Format Management articles.

You can also change the defaults for Doctrine so that the generated SQL uses the correct character set. To achieve this, put the following configuration into the config/config.yml file:

        charset: utf8mb4
            charset: utf8mb4
            collate: utf8mb4_unicode_ci


If you use the version of MySQL that is older than 5.7, also add the row_format: DYNAMIC option to the default_table_options section.

For more details, please see the Setting up the Database to be UTF8 article.

Block Nested Loop (BNL) in MySQL 5.6 and later

MySQL 5.6 has a lot of improvements in the query optimizer, but unfortunately, some queries work significantly slower than in MySQL 5.5. In case of large amounts of data in the database, it is possible to encounter performance issues related to the new query optimizer. One of the possible solutions might be disabling block_nested_loop option. This can be achieved in two ways:

  • If a modification of MySQL configuration file is granted, add the following under the mysqld section

  • Another way is to add the following configuration to the config/config.yml file

                        # PDO::MYSQL_ATTR_INIT_COMMAND
                        1002: 'SET @@SESSION.optimizer_switch="block_nested_loop=off";'