慢查詢日志概念

MySQL的慢查詢日志是MySQL提供的一種日志記錄，它用來記錄在MySQL中響應時間超過閥值的語句，具體指運行時間超過long_query_time值的SQL，則會被記錄到慢查詢日志中。long_query_time的默認值為10，意思是運行10S以上的語句。默認情況下，Mysql數據庫并不啟動慢查詢日志，需要我們手動來設置這個參數，當然，如果不是調優需要的話，一般不建議啟動該參數，因為開啟慢查詢日志會或多或少帶來一定的性能影響。慢查詢日志支持將日志記錄寫入文件，也支持將日志記錄寫入數據庫表。

官方文檔，關于慢查詢的日志介紹如下(部分資料，具體參考官方相關鏈接)：

The slow query log consists of SQL statements that took more than?long_query_time?seconds to execute and required at least?min_examined_row_limit?rows to be examined. The minimum and default values of?long_query_time?are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored.

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed usinglog_slow_admin_statements and log_queries_not_using_indexes, as described later.

慢查詢日志相關參數

MySQL 慢查詢的相關參數解釋：

slow_query_log??? ：是否開啟慢查詢日志，1表示開啟，0表示關閉。

log-slow-queries? ：舊版(5.6以下版本)MySQL數據庫慢查詢日志存儲路徑。可以不設置該參數，系統則會默認給一個缺省的文件host_name-slow.log

slow-query-log-file：新版(5.6及以上版本)MySQL數據庫慢查詢日志存儲路徑。可以不設置該參數，系統則會默認給一個缺省的文件host_name-slow.log

long_query_time ：慢查詢閾值，當查詢時間多于設定的閾值時，記錄日志。

log_queries_not_using_indexes：未使用索引的查詢也被記錄到慢查詢日志中(可選項)。

log_output：日志存儲方式。log_output='FILE'表示將日志存入文件，默認值是'FILE'。log_output='TABLE'表示將日志存入數據庫，這樣日志信息就會被寫入到mysql.slow_log表中。MySQL數據庫支持同時兩種日志存儲方式，配置的時候以逗號隔開即可，如：log_output='FILE,TABLE'。日志記錄到系統的專用日志表中，要比記錄到文件耗費更多的系統資源，因此對于需要啟用慢查詢日志，又需要能夠獲得更高的系統性能，那么建議優先記錄到文件。

慢查詢日志配置

默認情況下slow_query_log的值為OFF，表示慢查詢日志是禁用的，可以通過設置slow_query_log的值來開啟，如下所示：

mysql> show variables? like?'%slow_query_log%';

+---------------------+-----------------------------------------------+

| Variable_name?????? | Value???????????????????????????????????????? |

+---------------------+-----------------------------------------------+

| slow_query_log????? | OFF?????????????????????????????????????????? |

| slow_query_log_file | /home/WDPM/MysqlData/mysql/DB-Server-slow.log |

+---------------------+-----------------------------------------------+

2 rows?in?set (0.00 sec)

mysql> set?global slow_query_log=1;

Query OK, 0 rows affected (0.09 sec)

mysql> show variables like?'%slow_query_log%';

+---------------------+-----------------------------------------------+

| Variable_name?????? | Value???????????????????????????????????????? |

+---------------------+-----------------------------------------------+

| slow_query_log????? | ON??????????????????????????????????????????? |

| slow_query_log_file | /home/WDPM/MysqlData/mysql/DB-Server-slow.log |

+---------------------+-----------------------------------------------+

2 rows?in?set (0.00 sec)

mysql>

使用set global slow_query_log=1開啟了慢查詢日志只對當前數據庫生效，如果MySQL重啟后則會失效。如果要永久生效，就必須修改配置文件my.cnf(其它系統變量也是如此)。例如如下所示：

mysql> show variables like?'slow_query%';

+---------------------+-----------------------------------------------+

| Variable_name?????? | Value???????????????????????????????????????? |

+---------------------+-----------------------------------------------+

| slow_query_log????? | OFF?????????????????????????????????????????? |

| slow_query_log_file | /home/WDPM/MysqlData/mysql/DB-Server-slow.log |

+---------------------+-----------------------------------------------+

2 rows?in?set (0.01 sec)

mysql>

修改my.cnf文件，增加或修改參數slow_query_log 和slow_query_log_file后，然后重啟MySQL服務器，如下所示

slow_query_log =1

slow_query_log_file=/tmp/mysql_slow.log

mysql> show variables like?'slow_query%';

+---------------------+---------------------+

| Variable_name?????? | Value?????????????? |

+---------------------+---------------------+

| slow_query_log????? | ON????????????????? |

| slow_query_log_file | /tmp/mysql_slow.log |

+---------------------+---------------------+

2 rows?in?set (0.00 sec)

mysql>

關于慢查詢的參數slow_query_log_file ，它指定慢查詢日志文件的存放路徑，系統默認會給一個缺省的文件host_name-slow.log(如果沒有指定參數slow_query_log_file的話)

mysql> show variables like?'slow_query_log_file';

+---------------------+-----------------------------------------------+

| Variable_name?????? | Value???????????????????????????????????????? |

+---------------------+-----------------------------------------------+

| slow_query_log_file | /home/WDPM/MysqlData/mysql/DB-Server-slow.log |

+---------------------+-----------------------------------------------+

1 row?in?set (0.00 sec)

那么開啟了慢查詢日志后，什么樣的SQL才會記錄到慢查詢日志里面呢？ 這個是由參數long_query_time控制，默認情況下long_query_time的值為10秒，可以使用命令修改，也可以在my.cnf參數里面修改。關于運行時間正好等于long_query_time的情況，并不會被記錄下來。也就是說，在mysql源碼里是判斷大于long_query_time，而非大于等于。從MySQL 5.1開始，long_query_time開始以微秒記錄SQL語句運行時間，之前僅用秒為單位記錄。如果記錄到表里面，只會記錄整數部分，不會記錄微秒部分。

mysql> show variables like?'long_query_time%';

+-----------------+-----------+

| Variable_name?? | Value???? |

+-----------------+-----------+

| long_query_time | 10.000000 |

+-----------------+-----------+

1 row?in?set (0.00 sec)

mysql> set?global long_query_time=4;

Query OK, 0 rows affected (0.00 sec)

mysql> show variables like?'long_query_time';

+-----------------+-----------+

| Variable_name?? | Value???? |

+-----------------+-----------+

| long_query_time | 10.000000 |

+-----------------+-----------+

1 row?in?set (0.00 sec)

如上所示，我修改了變量long_query_time，但是查詢變量long_query_time的值還是10，難道沒有修改到呢？注意：使用命令 set global long_query_time=4修改后，需要重新連接或新開一個會話才能看到修改值。你用show variables like 'long_query_time'查看是當前會話的變量值，你也可以不用重新連接會話，而是用show global variables like 'long_query_time'; 如下所示：

在MySQL里面執行下面SQL語句，然后我們去檢查對應的慢查詢日志，就會發現類似下面這樣的信息。

mysql> select sleep(3);

+----------+

| sleep(3) |

+----------+

|??????? 0 |

+----------+

1 row?in?set (3.00 sec)

[root@DB-Server ~]# more /tmp/mysql_slow.log

/usr/sbin/mysqld, Version: 5.6.20-enterprise-commercial-advanced-log (MySQL Enterprise Server - Advanced Edition (Commercial)). started with:

Tcp port: 0? Unix socket: (null)

Time???????????????? Id Command??? Argument

/usr/sbin/mysqld, Version: 5.6.20-enterprise-commercial-advanced-log (MySQL Enterprise Server - Advanced Edition (Commercial)). started with:

Tcp port: 0? Unix socket: (null)

Time???????????????? Id Command??? Argument

# Time: 160616 17:24:35

# User@Host: root[root] @ localhost []? Id:???? 5

# Query_time: 3.002615? Lock_time: 0.000000 Rows_sent: 1? Rows_examined: 0

SET?timestamp=1466069075;

select sleep(3);

log_output 參數是指定日志的存儲方式。log_output='FILE'表示將日志存入文件，默認值是'FILE'。log_output='TABLE'表示將日志存入數據庫，這樣日志信息就會被寫入到mysql.slow_log表中。MySQL數據庫支持同時兩種日志存儲方式，配置的時候以逗號隔開即可，如：log_output='FILE,TABLE'。日志記錄到系統的專用日志表中，要比記錄到文件耗費更多的系統資源，因此對于需要啟用慢查詢日志，又需要能夠獲得更高的系統性能，那么建議優先記錄到文件。

mysql> show variables like?'%log_output%';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| log_output??? | FILE? |

+---------------+-------+

1 row?in?set (0.00 sec)

mysql> set?global log_output='TABLE';

Query OK, 0 rows affected (0.00 sec)

mysql> show variables like?'%log_output%';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| log_output??? | TABLE |

+---------------+-------+

1 row?in?set (0.00 sec)

mysql> select sleep(5) ;

+----------+

| sleep(5) |

+----------+

|??????? 0 |

+----------+

1 row?in?set (5.00 sec)

mysql>

mysql> select * from mysql.slow_log;

+---------------------+---------------------------+------------+-----------+-----------+---------------+----+----------------+-----------+-----------+-----------------+-----------+

| start_time????????? | user_host???????????????? | query_time | lock_time | rows_sent | rows_examined | db | last_insert_id | insert_id | server_id | sql_text??????? | thread_id |

+---------------------+---------------------------+------------+-----------+-----------+---------------+----+----------------+-----------+-----------+-----------------+-----------+

| 2016-06-16 17:37:53 | root[root] @ localhost [] | 00:00:03?? | 00:00:00? |???????? 1 |???????????? 0 |??? |????????????? 0 |???????? 0 |???????? 1 | select sleep(3) |???????? 5 |

| 2016-06-16 21:45:23 | root[root] @ localhost [] | 00:00:05?? | 00:00:00? |???????? 1 |???????????? 0 |??? |????????????? 0 |???????? 0 |???????? 1 | select sleep(5) |???????? 2 |

+---------------------+---------------------------+------------+-----------+-----------+---------------+----+----------------+-----------+-----------+-----------------+-----------+

2 rows?in?set (0.00 sec)

mysql>

系統變量log-queries-not-using-indexes：未使用索引的查詢也被記錄到慢查詢日志中(可選項)。如果調優的話，建議開啟這個選項。另外，開啟了這個參數，其實使用full index scan的sql也會被記錄到慢查詢日志。

This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows.

mysql> show variables like?'log_queries_not_using_indexes';

+-------------------------------+-------+

| Variable_name???????????????? | Value |

+-------------------------------+-------+

| log_queries_not_using_indexes | OFF?? |

+-------------------------------+-------+

1 row?in?set (0.00 sec)

mysql> set?global log_queries_not_using_indexes=1;

Query OK, 0 rows affected (0.00 sec)

mysql> show variables like?'log_queries_not_using_indexes';

+-------------------------------+-------+

| Variable_name???????????????? | Value |

+-------------------------------+-------+

| log_queries_not_using_indexes | ON??? |

+-------------------------------+-------+

1 row?in?set (0.00 sec)

mysql>

系統變量log_slow_admin_statements表示是否將慢管理語句例如ANALYZE TABLE和ALTER TABLE等記入慢查詢日志

mysql> show variables like?'log_slow_admin_statements';

+---------------------------+-------+

| Variable_name???????????? | Value |

+---------------------------+-------+

| log_slow_admin_statements | OFF?? |

+---------------------------+-------+

1 row?in?set (0.00 sec)

mysql>

系統變量log_slow_slave_statements 表示

By default, a replication slave does not write replicated queries to the slow query log. To change this, use thelog_slow_slave_statements system variable.

When the slow query log is enabled, this variable enables logging for queries that have taken more than long_query_time seconds to execute on the slave. This variable was added in MySQL 5.7.1. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START SLAVE statements.

參數--log-short-format

The server writes less information to the slow query log if you use the --log-short-format option.

Command-Line Format

--log-short-format

Permitted Values

Type

boolean

Default

FALSE

另外，如果你想查詢有多少條慢查詢記錄，可以使用系統變量。

mysql> show global status like?'%Slow_queries%';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| Slow_queries? | 2104? |

+---------------+-------+

1 row?in?set (0.00 sec)

mysql>

日志分析工具mysqldumpslow

在生產環境中，如果要手工分析日志，查找、分析SQL，顯然是個體力活，MySQL提供了日志分析工具mysqldumpslow

查看mysqldumpslow的幫助信息：

[root@DB-Server ~]# mysqldumpslow --help

Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]

Parse and summarize the MySQL slow query log. Options are

--verbose??? verbose

--debug????? debug

--help?????? write this text to standard output

-v?????????? verbose

-d?????????? debug

-s ORDER???? what to sort by (al, at, ar, c, l, r, t), 'at' is default

al: average lock time

ar: average rows sent

at: average query time

c: count

l: lock time

r: rows sent

t: query time

-r?????????? reverse the sort order (largest last instead of first)

-t NUM?????? just show the top n queries

-a?????????? don't abstract all numbers to N and strings to 'S'

-n NUM?????? abstract numbers with at least n digits within names

-g PATTERN?? grep: only consider stmts that include this string

-h HOSTNAME? hostname of db server for *-slow.log filename (can be wildcard),

default is '*', i.e. match all

-i NAME????? name of server instance (if using mysql.server startup script)

-l?????????? don't subtract lock time from total time

-s, 是表示按照何種方式排序，

c: 訪問計數

l: 鎖定時間

r: 返回記錄

t: 查詢時間

al:平均鎖定時間

ar:平均返回記錄數

at:平均查詢時間

-t, 是top n的意思，即為返回前面多少條的數據；

-g, 后邊可以寫一個正則匹配模式，大小寫不敏感的；

比如

得到返回記錄集最多的10個SQL。

mysqldumpslow -s r -t 10 /database/mysql/mysql06_slow.log

得到訪問次數最多的10個SQL

mysqldumpslow -s c -t 10 /database/mysql/mysql06_slow.log

得到按照時間排序的前10條里面含有左連接的查詢語句。

mysqldumpslow -s t -t 10 -g “left join” /database/mysql/mysql06_slow.log

另外建議在使用這些命令時結合 | 和more 使用 ，否則有可能出現刷屏的情況。

mysqldumpslow -s r -t 20 /mysqldata/mysql/mysql06-slow.log | more

參考資料：

https://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_long_query_time

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