Databases, Systems & Networks

To show/list the users in a MySQL database, first log into your MySQL server as an administrative user, then run this MySQL query:

select * from mysql.user;

This MySQL query shows a large listing of MySQL user information, including user permission information, so you may want to trim down some of the fields to display. You can get a listing of the fields in the mysql.user table by running this command:

desc mysql.user;

Lire la suite…

Pour charger une fichier texte défini comme suit :

$ tail /home/user1/test.txt
   'nom1',1,9
   'nom2',2,3
   'nom3',3,54
   'nom4',4,2
   'nom5',5,9

Dans une table définie comme suit :

CREATE TABLE chargertest (
                cle_prim int(11) NOT NULL auto_increment,
                nom varchar(20),
                x integer,
                y integer,
                z timestamp(14),
                Constraint pk_chargertest PRIMARY KEY  (cle_prim)
);

A noter que le champ ‘z’ n’est pas défini au niveau du fichier texte et que le séparateur utilisé est ‘,’.

mysql> load data infile '/home/user1/test.txt' into table chargertest fields terminated by ',' (nom,x,y);

Pour supprimer des doublons au niveau d’une table donnée définie comme suit :

CREATE TABLE IF NOT EXISTS TabTest (
           cle_prim integer(4) NOT NULL auto_increment,
           x integer,
           y integer,
           z integer,
           Constraint pk_Tab_test PRIMARY KEY  (cle_prim)
);

Il faut commencer par fixer les champs relatifs au doublons (dans notre cas les champs x et y):

mysql> select * from TabTest;
+----------+------+------+------+
| cle_prim | x    | y    | z    |
+----------+------+------+------+
|        1 |    1 |    2 |    3 |
|        2 |    1 |    2 |    3 |
|        3 |    1 |    5 |    4 |
|        4 |    1 |    6 |    4 |
+----------+------+------+------+
4 rows in set (0.00 sec)

Pour supprimer les doublons au niveau des champs x et y lancer la commande :

ALTER IGNORE TABLE  TabTest ADD UNIQUE INDEX(x,y);

Il suffit d’utiliser la clause DISTINCT entre SELECT et les champs.

Exemple :

SELECT DISTINCT id,nom,prenom FROM matable

Running MySQL at optimal settings for specific resources helps handle larger server loads and prevents server slowdown. Generally, after tuning Apache to handle larger loads, it is beneficial to tune MySQL to additional connections.

Database tuning is an expansive topic, and this guide covers only the basics of editing your MySQL configuration. Large MySQL databases can require a considerable amount of memory. For this reason, we recommend using a high memory Linode for such setups.

The steps in this guide require root privileges. Be sure to run the steps below as root or with the sudo prefix. For more information on privileges see our Users and Groups guide.

Tools That Can Help Optimize MySQL

In order to determine if your MySQL database needs to be reconfigured, it is best to look at how your resources are performing now. This can be done with the top command or with the Linode Longview service. At the very least, you should familiarize yourself with the RAM and CPU usage of your server, which can be discovered with these commands:

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echo [PID]  [MEM]  [PATH] &&  ps aux | awk '{print $2, $4, $11}' | sort -k2rn | head -n 20
ps -eo pcpu,pid,user,args | sort -k 1 -r | head -20

MySQLTuner

The MySQLTuner script assesses your MySQL installation, and then outputs suggestions for increasing your server’s performance and stability.

1. Download and run MySQLTuner:

   1	curl -L http://mysqltuner.pl/ | perl

2. It outputs your results:

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   >> MySQLTuner 1.4.0 - Major Hayden <major@mhtx.net> >> Bug reports, feature requests, and downloads at http://mysqltuner.com/ >> Run with '--help' for additional options and output filtering Please enter your MySQL administrative login: root Please enter your MySQL administrative password: [OK] Currently running supported MySQL version 5.5.41-0+wheezy1 [OK] Operating on 64-bit architecture -------- Storage Engine Statistics ------------------------------------------- [--] Status: +ARCHIVE +BLACKHOLE +CSV -FEDERATED +InnoDB +MRG_MYISAM [--] Data in InnoDB tables: 1M (Tables: 11) [--] Data in PERFORMANCE_SCHEMA tables: 0B (Tables: 17) [!!] Total fragmented tables: 11 -------- Security Recommendations ------------------------------------------- [OK] All database users have passwords assigned -------- Performance Metrics ------------------------------------------------- [--] Up for: 47s (113 q [2.404 qps], 42 conn, TX: 19K, RX: 7K) [--] Reads / Writes: 100% / 0% [--] Total buffers: 192.0M global + 2.7M per thread (151 max threads) [OK] Maximum possible memory usage: 597.8M (60% of installed RAM) [OK] Slow queries: 0% (0/113) [OK] Highest usage of available connections: 0% (1/151) [OK] Key buffer size / total MyISAM indexes: 16.0M/99.0K [!!] Query cache efficiency: 0.0% (0 cached / 71 selects) [OK] Query cache prunes per day: 0 [OK] Temporary tables created on disk: 25% (54 on disk / 213 total) [OK] Thread cache hit rate: 97% (1 created / 42 connections) [OK] Table cache hit rate: 24% (52 open / 215 opened) [OK] Open file limit used: 4% (48/1K) [OK] Table locks acquired immediately: 100% (62 immediate / 62 locks) [OK] InnoDB buffer pool / data size: 128.0M/1.2M [OK] InnoDB log waits: 0 -------- Recommendations ----------------------------------------------------- General recommendations: Run OPTIMIZE TABLE to defragment tables for better performance Enable the slow query log to troubleshoot bad queries Variables to adjust: query_cache_limit (> 1M, or use smaller result sets)

   MySQLTuner offers suggestions regarding how to better the database’s performance. If you are wary about updating your database on your own, following MySQLTuner’s suggestions is one of the safer ways to improve your database performance.

Tuning MySQL

When altering the MySQL configuration, be alert to the changes and how they affect your database. Even when following the instructions of programs such as MySQLTuner, it is best to have some understanding of the process.

The file you are changing is located at /etc/mysql/my.cnf.

Prior to updating the MySQL configuration, create a backup of the my.cnf file:

1	cp /etc/mysql/my.cnf ~/my.cnf.backup

Best practice suggests that you make small changes, one at a time, and then monitor the server after each change. You should restart MySQL after each change:

• For systems without systemd:

  1	systemctl restart mysqld

• For distributions which don’t use systemd:

  1	service mysql restart

When changing values in the my.cnf file, be sure that the line you are changing hasn’t been commented out with the pound (#) prefix.

key_buffer

Changing the key_buffer allocates more memory to MySQL, which can substantially speed up your databases, assuming you have the memory free. The key_buffer size should generally take up no more than 25 percent of the system memory when using the MyISAM table engine, and up to 70 percent for InnoDB. If the value is set too high, resources are wasted.

According to MySQL’s documentation, for servers with 256MB (or more) of RAM with many tables, a setting of 64M is recommended. Servers with 128MB of RAM and fewer tables can be set to 16M, the default value. Websites with even fewer resources and tables can have this value set lower.

max_allowed_packet

This parameter lets you set the maximum size of a sendable packet. A packet is a single SQL state, a single row being sent to a client, or a log being sent from a master to a slave. If you know that your MySQL server is going to be processing large packets, it is best to increase this to the size of your largest packet. Should this value be set too small, you would receive an error in your error log.

thread_stack

This value contains the stack size for each thread. MySQL considers the default value of the thread_stack variable sufficient for normal use; however, should an error relating to the thread_stack be logged, this can be increased.

thread_cache_size

If thread_cache_size is “turned off” (set to 0), then any new connection being made needs a new thread created for it. When the connections disengage the thread is destroyed. Otherwise, this value sets the number of unused threads to store in a cache until they need to be used for a connection. Generally this setting has little affect on performance, unless you are receiving hundreds of connections per minute, at which time this value should be increased so the majority of connections can be made on cached threads.

max_connections

This parameter sets the maximum amount of concurrent connections. It is best to consider the maximum amount of connections you have had in the past before setting this number, so you’ll have a buffer between that upper number and the max_connections value. Note, this does not indicate the maximum amount of users on your website at one time; rather it shows the maximum amount of users making requests concurrently.

table_cache

This value should be kept higher than your open_tables value. To determine this value use:

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SHOW STATUS LIKE 'open%';