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(Errcode: 24 – Too many open files) [23] – Plesk Onyx 17.5.3 and Ubuntu 16.04.2 LTS‬

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As of MySQL 5.7.7, this is what the documentation recommends for Red Hat Enterprise Linux 7, Oracle Linux 7, CentOS 7, SUSE Linux Enterprise Server 12, Fedora 24 and 25:

https://dev.mysql.com/doc/refman/5.7/en/using-systemd.html

On Ubuntu 16.04 the service is called mysql, not mysqld, so this is what I did:

sudo mkdir /etc/systemd/system/mysql.service.d
sudo vi /etc/systemd/system/mysql.service.d/override.conf
Added this in the new file override.conf:

[Service]
LimitNOFILE=1024 4096

Then restarted the service:

sudo systemctl daemon-reload
sudo systemctl restart mysql

and test it with

cat /proc/$(pgrep mysql)/limits | grep files

Posted in Linux, PLESK 26 Comments

Partitioning on MySql

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Imagine you need to execute one ore more queries on a big size table, containing the history of the sales of an e-commerce point.

If your hardware resources are limited, scanning the whole table could require several minutes, and this in turn could imply long table locks and waste of time to the users.

Since version 5.1, MySql supports partitioning, a mechanism allowing data to be divided according to access needs.

There are several partitioning types. The most popular mechanisms are:

– RANGE: rows are divided based upon a specified column range

– HASH: the column hash is calculated based upon the record position in the partition resulting from the operation

– LIST: works like RANGE but the values are not surely adjacent

The partitioning of a table works in two different manners:

– during the table creation

CREATE TABLE sales (
    id int NOT NULL,
    order_date DATETIME NOT NULL,
    user bigint NOT NULL,
    total float NOT NULL DEFAULT '0',
    receipt_id bigint NULL
) ENGINE=InnoDB PARTITION BY RANGE(YEAR(order_date)) ( 
  PARTITION p_history VALUES LESS THAN (2014), 
  PARTITION p_data VALUES LESS THAN MAXVALUE 
);

during the modification of the table structure

ALTER TABLE sales PARTITION BY RANGE(YEAR(order_date)) (
	PARTITION p_history VALUES LESS THAN (2014),
	PARTITION p_data VALUES LESS THAN MAXVALUE
);

An important tool for debugging the extraction operations is the following command

EXPLAIN PARTITIONS;

The command output shows the partition used by MySql for execution of the specified query.

The partitioning is a very effective tool, but it may give no results, if you don’t choose the proper partitioning strategy: partitioning a sales table according to the creation date and then filtering data based on users, might be useless, independently on the partition criteria, or could even worst the situation.

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Event scheduler in MySql

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Since Mysql version 5.1.6, the system supports the option to schedule the execution of some events, without need to use external software (cron, at, Task Scheduler, etc.)  or to change data as required for trigger implementation.

To activate the schedule function in MySql, you have simply to execute in a client the following command:

SET GLOBAL event_scheduler = 1;

Then you can verify the correct system working, by executing the following command:

SHOW PROCESSLIST;

A task can be executed once or more times and can be defined via a SQL syntax.

CREATE EVENT DELETE_BASKET_DATA
  ON SCHEDULE EVERY 1 MINUTE 
  DO DELETE FROM basket where created > DATE_SUB(NOW(), INTERVAL 30 MINUTE);

When a task is executed, all data older than 30 minutes will be deleted.

To show how many and which tasks have been defined on the database, you need to simply execute the command “SHOW EVENTS”.

To erase a task, you can simply execute the following command

DROP EVENT DELETE_BASKET_DATA;

By using this function, you can perform some maintenance tasks on data, without affecting other parts of your application. With such options, MySql was able to fill its gap with some of the most important relational databases currently available.

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