本文例項講述了mysql慢查詢操作。分享給大家供大家參考，具體如下：

mysql有些sql會執行很慢，有可能造成伺服器負載飆升

首先查詢 確定影響負載的是mysql ，使用top命令，ps命令等

其次，進入MySQL，使用show full processlist查詢執行中的sql語句，看看問題，使用explain 命令 檢視狀態

最後找出sql語句殺死或者優化

centos7上面安裝mariadb服務

yum -y install mariadb-server mariadb-devel

開啟慢查詢

more /etc/my.cnf.d/server.cnf

[mariadb]
slow_query_log=ON
slow_query_log_file=/usr/local/mysql/data/slow.log
long_query_time=1

啟動mariadb服務

systemctl start mariadb

查詢mysql的慢查詢是否開啟，以及多久的時間以上是慢查詢

MariaDB [(none)]> show variables like '%slow_query%';
+---------------------+--------------------------------+
| Variable_name    | Value             |
+---------------------+--------------------------------+
| slow_query_log   | ON               |
| slow_query_log_file | /usr/local/mysql/data/slow.log |
+---------------------+--------------------------------+
2 rows in set (0.00 sec)

MariaDB [(none)]> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name  | Value  |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

#如果沒用開啟慢查詢，可以在命令列開啟
mysql> set global slow_query_log=1;
Query OK,0 rows affected (0.00 sec)

測試慢查詢，以及檢視日誌

MariaDB [(none)]> select sleep(2);
+----------+
| sleep(2) |
+----------+
|    0 |
+----------+
1 row in set (2.00 sec)

[root@localhost ~]# more /usr/local/mysql/data/slow.log
/usr/libexec/mysqld,Version: 5.5.60-MariaDB (MariaDB Server). started with:
Tcp port: 0 Unix socket: /var/lib/mysql/mysql.sock
Time         Id Command  Argument
# Time: 180930 23:51:07
# User@Host: root[root] @ localhost []
# Thread_id: 2 Schema:  QC_hit: No
# Query_time: 2.001017 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1538322667;
select sleep(2);

確認慢查詢

MariaDB [(none)]> show full processlist; #檢視state慢查詢在進行
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
| Id | User | Host   | db  | Command | Time | State   | Info         | Progress |
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
| 3 | root | localhost | NULL | Query  |  9 | User sleep | select sleep(10)   |  0.000 |
| 4 | root | localhost | NULL | Query  |  0 | NULL    | show full processlist |  0.000 |
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
2 rows in set (0.00 sec)

MariaDB [(none)]> show full processlist; #檢視state慢查詢已經結束，但是使用者登陸了
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
| Id | User | Host   | db  | Command | Time | State | Info         | Progress |
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
| 3 | root | localhost | NULL | Sleep  |  1 |    | NULL         |  0.000 |
| 4 | root | localhost | NULL | Query  |  0 | NULL | show full processlist |  0.000 |
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
2 rows in set (0.00 sec)

更多關於MySQL相關內容感興趣的讀者可檢視本站專題：《MySQL查詢技巧大全》、《MySQL常用函式大彙總》、《MySQL日誌操作技巧大全》、《MySQL事務操作技巧彙總》、《MySQL儲存過程技巧大全》及《MySQL資料庫鎖相關技巧彙總》

希望本文所述對大家MySQL資料庫計有所幫助。