MySQL日志——通用日志和慢查询日志[通俗易懂]

MySQL的通用日志：

用来记录对数据库的通用操作，包括错误的sql语句等信息。

通用日志可以保存在：file（默认值）或 table（mysql.general_log表）

mysql通用日志的设置：

general_log=ON|OFF  --- 是否启用通用日志

general_log_file=HOSTNAME.log ---#通用日志存放的文件路径

log_output=TABLE|FILE|NONE --- 通用日志的存放方式

范例：

#查看是否启用通用日志：
mysql> select @@general_log;
+---------------+
| @@general_log |
+---------------+
|             0 |
+---------------+
1 row in set (0.00 sec)

mysql> show variables like "general_log";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| general_log   | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

general_log是一个全局变量。

#查看通用日志的存放路径
mysql> select @@general_log_file;
+-------------------------+
| @@general_log_file      |
+-------------------------+
| /data/mysql/CentOS8.log |
+-------------------------+
1 row in set (0.00 sec)

hostname.log

#启用通用日志
mysql> set general_log=1;
ERROR 1229 (HY000): Variable "general_log" is a GLOBAL variable and should be set with SET GLOBAL

mysql> set global  general_log=1;
Query OK, 0 rows affected (0.00 sec)

#通用日志默认村存放在file中
mysql> show variables like "log_output";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output    | FILE  |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show global variables like "log_output";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output    | FILE  |
+---------------+-------+
1 row in set (0.00 sec)

#修改通用日志的默认存放

将通用日志存放在一张表中
mysql> set global log_output="table";
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE "log_output";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output    | TABLE |
+---------------+-------+
1 row in set (0.00 sec)

#改为存放到表以后，通用日志是存放在mysql数据库的gerneral_log这张表中的。
mysql> use mysql;
Database changed
mysql> show tables;
+---------------------------+
| Tables_in_mysql           |
+---------------------------+
| columns_priv              |
| db                        |
| event                     |
| func                      |
| general_log               |
| help_category             |
| help_keyword              |
| help_relation             |
| help_topic                |
| innodb_index_stats        |
| innodb_table_stats        |
| ndb_binlog_index          |
| plugin                    |
| proc                      |
| procs_priv                |
| proxies_priv              |
| servers                   |
| slave_master_info         |
| slave_relay_log_info      |
| slave_worker_info         |
| slow_log                  |
| tables_priv               |
| time_zone                 |
| time_zone_leap_second     |
| time_zone_name            |
| time_zone_transition      |
| time_zone_transition_type |
| user                      |
+---------------------------+
28 rows in set (0.00 sec)

#查看general_log表
mysql> select * from general_log;
+---------------------+---------------------------+-----------+-----------+--------------+---------------------------+
| event_time          | user_host                 | thread_id | server_id | command_type | argument                  |
+---------------------+---------------------------+-----------+-----------+--------------+---------------------------+
| 2022-09-17 12:41:03 | root[root] @ localhost [] |         2 |         0 | Query        | select * from general_log |
+---------------------+---------------------------+-----------+-----------+--------------+---------------------------+
1 row in set (0.00 sec)

mysql> show table status like "general_log"G

general_log这张表就是一个CSV（文本文件）中，
文件路径：mysql数据文件存放位置（默认/var/lib/mysql）里面的mysql目录中。

[root@CentOS8 mysql]# pwd
/data/mysql/mysql
[root@CentOS8 mysql]# cat general_log.CSV
"2022-09-17 12:41:03","root[root] @ localhost []",2,0,"Query","select * from general_log"
"2022-09-17 12:41:52","root[root] @ localhost []",2,0,"Query","show table status like "general_log""
"2022-09-17 12:41:56","root[root] @ localhost []",2,0,"Query","show table status like "general_log""
"2022-09-17 12:43:27","root[root] @ localhost []",2,0,"Query","select * from general_log"
"2022-09-17 12:45:57","root[root] @ localhost []",2,0,"Query","show table status like "general_log""

通用日志的作用：用来观察数据库发生的事件。

MySQL的慢查询日志

记录执行查询时长超出指定时长的操作

慢查询日志的相关设置；

#默认没有启用慢查询日志
slow_query_log=ON|OFF #开启或关闭慢查询，支持全局和会话，只有全局设置才会生成慢查询文件

long_query_time=N #慢查询的阀值，单位秒,默认为10s，超过这个时间就叫慢查询

slow_query_log_file=HOSTNAME-slow.log  #记录慢查询日志的文件

log_slow_filter = admin,filesort,filesort_on_disk,
		  full_join,full_scan,
		  query_cache,query_cache_miss,
		  tmp_table,tmp_table_on_disk
#上述查询类型且查询时长超过long_query_time，则记录日志 慢查询记录的相关行为

log_queries_not_using_indexes=ON  #执行sql语句的时候没有利用索引或者使用全索引扫描，不管是否超过阈值都要记录在慢查询日志中。（默认OFF，即不记录）

log_slow_rate_limit = 1 #多少次查询才记录，mariadb特有

log_slow_verbosity= Query_plan,explain #记录内容

log_slow_queries = OFF    #同slow_query_log，MariaDB 10.0/MySQL 5.6.1 版后已删除

#开启慢查询
mysql> set global slow_query_log=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@slow_query_log;
+------------------+
| @@slow_query_log |
+------------------+
|                1 |
+------------------+
1 row in set (0.00 sec)

#修改慢查询的默认值
mysql> set long_query_time=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@long_query_time;
+-------------------+
| @@long_query_time |
+-------------------+
|          1.000000 |
+-------------------+
1 row in set (0.00 sec)

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