慢SQL探秘之为什么我的SQL很慢却没记录在慢查询日志里

吾爱主题阅读：172 2024-03-25 16:39:00 评论：0

在MySQL数据库中，想了解数据库运行情况的重要指标之一是慢SQL。而并非如某些人所说的所有运行慢的SQL都会被记录在慢SQL日志（或日志表）里，抑或是没有慢SQL就代表没有运行慢的SQL。本文将总结一些比较常见的运行比较慢但不会被记录在慢SQL日志里的情况。另外，慢SQL的计算方式在MySQL8.0新版本中有变化，因此，将通过对比MySQL5.7（MySQL5.7.38）与MySQL8.0（MySQL8.0.33）进行总结。

1. 准备工作

部署了两套环境，分别是MySQL5.7（MySQL5.7.38）版本及MySQL8.0（MySQL8.0.33）版本。另外为了后续进行慢SQL测试，此时先创建一张测试表并清空慢SQL日志表。

（1）创建测试表及数据

创建测试表及测试数据，便于后续测试。本次通过创建一张1000W记录的表进行测试。

然后再添加个字段。

mysql> call sp_createNum(10000000);
Query OK, 1611392 rows affected (38.70 sec)
mysql> select  count(*) from  testdb.nums;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (3.70 sec)
mysql> alter table testdb.nums add c1 varchar(20);
Query OK, 0 rows affected (17.83 sec)
Records: 0  Duplicates: 0  Warnings: 0

（2）清空慢SQL日志表

测试前先清空慢SQL日志表mysql.slow_log，清空方法如下：

mysql> select  count(*) from mysql.slow_log;
+----------+
| count(*) |
+----------+
|        2 |
+----------+
1 row in set (0.00 sec)
# 需先关闭慢SQL监控开关
mysql> set global slow_query_log=0;
Query OK, 0 rows affected (0.00 sec)
# truncate 方式清空慢SQL日志表
mysql> truncate table  mysql.slow_log;
Query OK, 0 rows affected (0.00 sec)
mysql> select  count(*) from mysql.slow_log;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)
# 清理完毕后开启慢SQL监控
mysql> set global slow_query_log=1;
Query OK, 0 rows affected (0.00 sec)

2. 未开启慢SQL监控

查看MySQL是否开启MySQL的方法如下：

mysql> SHOW GLOBAL VARIABLES LIKE 'slow_query_log';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| slow_query_log | ON    |
+----------------+-------+
1 row in set (0.00 sec)

其中value值为ON （或1），则代表开启了慢SQL监控。MySQL各个版本查看的方法均一样。

另外和慢SQL相关的其他主要参数如下：

slow_query_log: 这个参数用于启用或禁用慢SQL监控。设置为1表示启用，0表示禁用。默认值为0（禁用）。
log_output：日志存储方式（不仅仅是慢SQL日志），默认值为'FILE'。当log_output='FILE'表示将日志存入文件；当log_output='TABLE'表示将日志存入数据库中的mysql.slow_log表里；当log_output='FILE,TABLE'表示既存储到日志文件又存储到mysql.slow_log表里。
slow_query_log_file: 慢SQL日志文件的路径和文件名（5.5等低版本参数为log_slow_queries）。可以不设置该参数，系统则会默认给一个缺省的文件host_name-slow.log。
long_query_time: 用于定义慢SQL的阈值时间，单位为秒。执行时间超过该阈值的SQL语句将被记录到慢SQL日志中。默认值为10秒。
log_queries_not_using_indexes:如果设置为1，则将未使用索引的查询也记录到慢查询日志中。默认值为0（禁用）。
log_slow_admin_statements: 如果设置为1，则会记录部分管理命令（例如ALTER TABLE）到慢SQL日志中。默认值为0（禁用）,本文后续也会继续演示介绍。
log_slow_extra: 如果设置为1，则除了慢SQL日志的标准输出之外，还将在日志中包括额外的信息，如用户、主机、客户端命令等。默认值为0（禁用）。
log_slow_slave_statements: 如果设置为1，则将从服务器执行的慢SQL记录到主服务器的慢SQL日志中。默认值为0（禁用）。
min_examined_row_limit: 仅在查询的行数超过指定值时，才记录到慢SQL日志中。默认值为0，表示不限制。

3. SQL运行时间小于慢SQL监控阈值时间

第一部分已经介绍了和慢SQL相关的参数中的long_query_time，即慢SQL阈值。所以，当SQL运行时间小于该阈值时，对于的SQL将不会记录在慢SQL日志中。查看和修改慢SQL监控阈值的方法如下：

# 查看慢SQL阈值
mysql> SHOW GLOBAL VARIABLES LIKE 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 0.500000 |
+-----------------+----------+
1 row in set (0.00 sec)
# 设置慢SQL阈值
mysql> set global long_query_time=0.6;
Query OK, 0 rows affected (0.00 sec)
# 设置完成后可以查看全局的阈值
mysql> SHOW GLOBAL VARIABLES LIKE 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 0.600000 |
+-----------------+----------+
1 row in set (0.01 sec)
# 但是当前会话的慢SQL阈值是没变的，这个同其他包含全局和会话级的参数类似
mysql> SHOW  VARIABLES LIKE 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 0.500000 |
+-----------------+----------+
1 row in set (0.00 sec)

注：对于不同的数据库需按照实际情况设置慢SQL监控的阈值，例如TP业务的实例且配置相对较好时，建议阈值设置的较低；如果是AP类型业务，则适当放宽慢SQL的阈值。

4. 锁等待或事务等待的SQL

开启2个事务，然后模拟锁等待情况。

（1）MySQL5.7 中测试

首先测试MySQL5.7版本的情况：

事务1	事务2
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:40:47 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<=5; Query OK, 5 rows affected (7.85 sec) Rows matched: 5 Changed: 5 Warnings: 0 mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:41:07 \| +---------------------+ 1 row in set (0.00 sec)
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:41:20 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<3; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:42:55 \| +---------------------+ 1 row in set (0.00 sec)
mysql> select * from mysql.slow_log\G ************************* 1. row ************************* start_time: 2024-03-24 20:41:03.204598 user_host: root[root] @ localhost [] query_time: 00:00:07.853949 lock_time: 00:00:00.000089 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: update testdb.nums set c1=id where id<=5 thread_id: 2 1 row in set (0.00 sec)	mysql> select * from mysql.slow_log\G ************************* 1. row ************************* start_time: 2024-03-24 20:41:03.204598 user_host: root[root] @ localhost [] query_time: 00:00:07.853949 lock_time: 00:00:00.000089 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: update testdb.nums set c1=id where id<=5 thread_id: 2 1 row in set (0.00 sec)

从测试情况来看，MySQL5.7的锁等待超时的SQL是没有被记录在慢SQL日志中的

（2）MySQL8.0中测试

事务1	事务2
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:59:20 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<=5; Query OK, 5 rows affected (12.67 sec) Rows matched: 5 Changed: 5 Warnings: 0
mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:00:01 \| +---------------------+ 1 row in set (0.00 sec) mysql> select ,CONVERT(sql_text USING utf8mb4)sql_text2 from mysql.slow_log\G ************************ 1. row ************************* start_time: 2024-03-24 20:59:55.819649 user_host: root[root] @ localhost [] query_time: 00:00:12.676771 lock_time: 00:00:00.000003 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C3D35 thread_id: 87 sql_text2: update testdb.nums set c1=id where id<=5
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:02:21 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<3; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
	mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:03:41 \| +---------------------+ 1 row in set (0.00 sec) mysql> select ,CONVERT(sql_text USING utf8mb4)sql_text2 from mysql.slow_log\G ************************ 1. row *********************** start_time: 2024-03-24 20:59:55.819649 user_host: root[root] @ localhost [] query_time: 00:00:12.676771 lock_time: 00:00:00.000003 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C3D35 thread_id: 87 sql_text2: update testdb.nums set c1=id where id<=5 *********************** 2. row ************************* start_time: 2024-03-24 21:03:31.882874 user_host: root[root] @ localhost [] query_time: 00:01:00.006259 lock_time: 00:01:00.005760 rows_sent: 0 rows_examined: 1 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C33 thread_id: 88 sql_text2: update testdb.nums set c1=id where id<3 2 rows in set (0.00 sec)

MySQL8.0中，锁等待超时的SQL也会被记录在慢SQL记录中了，这与MySQL8.0后续新版中慢SQL的计算方式有调整有关系。

5. 管理类SQL

管理类SQL指的是alter table、alter user等,默认情况下，此类操作虽然比较慢，超过了慢SQL日志监控的阈值，但是也不会记录在慢SQL日志中。不过可以调整参数log_slow_admin_statements来控制是否记录此类SQL。

（1）默认情况

mysql> use testdb;
Database changed
mysql> alter table testdb.nums add primary key (id);
Query OK, 0 rows affected (1 min 10.93 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> select * from mysql.slow_log
    -> \G
Empty set (0.00 sec)

此时，虽然加主键的SQL运行了1分钟以上，但是慢SQL日志表里无此记录。

MySQL8.0 中同样如此。

（2）调整log_slow_admin_statements

log_slow_admin_statements参数是控制记录超时的管理操作SQL是否记录到慢查询日志。默认情况下的值是0，也就是不记录；而将值改为1时，此类SQL将会被记录。

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


mysql> alter table testdb.nums add key idx_c1(c1);
Query OK, 0 rows affected (16.54 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> select * from mysql.slow_log\G
*************************** 1. row ***************************
    start_time: 2024-03-24 21:39:43.181950
     user_host: root[root] @ localhost []
    query_time: 00:00:16.545439
     lock_time: 00:00:00.001927
     rows_sent: 0
 rows_examined: 0
            db: testdb
last_insert_id: 0
     insert_id: 0
     server_id: 1
      sql_text: alter table testdb.nums add key idx_c1(c1)
     thread_id: 8
1 row in set (0.00 sec)


mysql> select version();
+---------------+
| version()     |
+---------------+
| 5.7.38-41-log |
+---------------+
1 row in set (0.00 sec)

此时，添加索引的操作将被记录。

MySQL8.0中同样适用。

6. 扫描记录少于阈值的SQL

MySQL中扫描记录少于阈值由min_examined_row_limit参数控制，默认值为0，即如果SQL扫描的行数少于此值时，将不会被记录在慢SQL日志中，否则将会被记录。由于默认值是0，因此扫描行数>=0的且符合其他记录慢SQL的条件时便会被记录。如果想忽略扫描数据量较少，但是又不想记录超过阈值的SQL，则可以调整min_examined_row_limit来解决。

（1）默认情况

测试一下默认情况.

mysql> show global variables like 'min_examined_row_limit';
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| min_examined_row_limit | 0     |
+------------------------+-------+
1 row in set (0.00 sec)
#c1<=999，可以隐式转换导致无法走索引，使其变慢，便于测试
mysql> select count(*) from testdb.nums where c1<=999;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (1.70 sec)
mysql> select * from mysql.slow_log\G
*************************** 1. row ***************************
    start_time: 2024-03-24 21:48:46.005622
     user_host: root[root] @ localhost []
    query_time: 00:00:01.691788
     lock_time: 00:00:00.000092
     rows_sent: 1
 rows_examined: 10000000
            db: testdb
last_insert_id: 0
     insert_id: 0
     server_id: 25455
      sql_text: select count(*) from testdb.nums where c1<=999
     thread_id: 8

此时慢SQL会被记录。MySQL8.0中同样如此。

（2）修改参数

为了测试，此时将min_examined_row_limit值设置为20000000，然后测试是否还会被记录。

mysql> set min_examined_row_limit=20000000;
Query OK, 0 rows affected (0.00 sec)
mysql> select count(*) from testdb.nums where c1<=999;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (1.70 sec)
mysql> select * from mysql.slow_log\G
*************************** 1. row ***************************
    start_time: 2024-03-24 21:48:46.005622
     user_host: root[root] @ localhost []
    query_time: 00:00:01.691788
     lock_time: 00:00:00.000092
     rows_sent: 1
 rows_examined: 10000000
            db: testdb
last_insert_id: 0
     insert_id: 0
     server_id: 25455
      sql_text: select count(*) from testdb.nums where c1<=999
     thread_id: 8
1 row in set (0.00 sec)

可见，此时的慢SQL还是之前的，即修改后，即使SQL运行时间超过了慢SQL阈值，但是扫描行数低于min_examined_row_limit参数指定的值，此时也不会被记录。MySQL同样如此。

7. 其他SQL

除了以上的情况外，复制线程的查询、被DBAkill的正在运行的SQL或部分未运行完毕的SQL也不会记录在慢SQL日志中（不过部分情况再MySQL8.0中有所变更），因此需要大家根据实际情况多总结及测试。

原文地址：https://mp.weixin.qq.com/s/cx3HKVb0XT0EBcQFvXRGAA

可以去百度分享获取分享代码输入这里。

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事务1	事务2
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:40:47 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<=5; Query OK, 5 rows affected (7.85 sec) Rows matched: 5 Changed: 5 Warnings: 0 mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:41:07 \| +---------------------+ 1 row in set (0.00 sec)
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:41:20 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<3; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:42:55 \| +---------------------+ 1 row in set (0.00 sec)
mysql> select * from mysql.slow_log\G ************************* 1. row ************************* start_time: 2024-03-24 20:41:03.204598 user_host: root[root] @ localhost [] query_time: 00:00:07.853949 lock_time: 00:00:00.000089 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: update testdb.nums set c1=id where id<=5 thread_id: 2 1 row in set (0.00 sec)	mysql> select * from mysql.slow_log\G ************************* 1. row ************************* start_time: 2024-03-24 20:41:03.204598 user_host: root[root] @ localhost [] query_time: 00:00:07.853949 lock_time: 00:00:00.000089 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: update testdb.nums set c1=id where id<=5 thread_id: 2 1 row in set (0.00 sec)

事务1	事务2
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 20:59:20 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<=5; Query OK, 5 rows affected (12.67 sec) Rows matched: 5 Changed: 5 Warnings: 0
mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:00:01 \| +---------------------+ 1 row in set (0.00 sec) mysql> select ,CONVERT(sql_text USING utf8mb4)sql_text2 from mysql.slow_log\G ************************ 1. row ************************* start_time: 2024-03-24 20:59:55.819649 user_host: root[root] @ localhost [] query_time: 00:00:12.676771 lock_time: 00:00:00.000003 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C3D35 thread_id: 87 sql_text2: update testdb.nums set c1=id where id<=5
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:02:21 \| +---------------------+ 1 row in set (0.00 sec) mysql> update testdb.nums set c1=id where id<3; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
	mysql> select now(); +---------------------+ \| now() \| +---------------------+ \| 2024-03-24 21:03:41 \| +---------------------+ 1 row in set (0.00 sec) mysql> select ,CONVERT(sql_text USING utf8mb4)sql_text2 from mysql.slow_log\G ************************ 1. row *********************** start_time: 2024-03-24 20:59:55.819649 user_host: root[root] @ localhost [] query_time: 00:00:12.676771 lock_time: 00:00:00.000003 rows_sent: 0 rows_examined: 10000000 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C3D35 thread_id: 87 sql_text2: update testdb.nums set c1=id where id<=5 *********************** 2. row ************************* start_time: 2024-03-24 21:03:31.882874 user_host: root[root] @ localhost [] query_time: 00:01:00.006259 lock_time: 00:01:00.005760 rows_sent: 0 rows_examined: 1 db: testdb last_insert_id: 0 insert_id: 0 server_id: 1 sql_text: 0x757064617465207465737464622E6E756D73207365742063313D69642077686572652069643C33 thread_id: 88 sql_text2: update testdb.nums set c1=id where id<3 2 rows in set (0.00 sec)