识别 MySQL 中的慢速查询

显示完整进程列表

要显示 MySQL 正在执行的当前操作的处理线程，请键入以下内容：

SHOW FULL PROCESSLIST;

 +----+-----------------+-----------+------+---------+---------+------------------------+-----------------------+
 | Id | User            | Host      | db   | Command | Time    | State                  | Info                  |
 +----+-----------------+-----------+------+---------+---------+------------------------+-----------------------+
 |  5 | event_scheduler | localhost | NULL | Daemon  | 1834441 | Waiting on empty queue | NULL                  |
 | 50 | root            | localhost | NULL | Query   |       0 | init                   | show full processlist |
 +----+-----------------+-----------+------+---------+---------+------------------------+-----------------------+
 2 rows in set (0.00 sec)

上面的输出显示了一个空闲服务器，其中只有我们自己的查询和一个长时间运行的事件监听器。活动服务器将显示更多进程，其中一些可能是长时间运行的。如果没有 `FULL` 修饰符，此命令将仅显示前 100 个进程，这可能会截断你的结果，具体取决于你的服务器活动。

需要关注的一些重要部分是 `Time` 和 `State` 列。`Time` 列计算线程处于所述 `State` 的秒数。如果你发现 `Time` 值不符合给定操作的预期，则可能需要进一步调查。

检查存储引擎状态

另一个检查的地方是实际存储引擎的状态。

你可以键入以下内容来查找与给定表关联的 存储引擎：

SHOW CREATE TABLE <database>.<table>\G

例如，要显示 `mysql.time_zone` 表使用的存储引擎，请键入以下内容：

SHOW CREATE TABLE mysql.time_zone`\G

*************************** 1. row ***************************
       Table: time_zone
Create Table: CREATE TABLE `time_zone` (
  `Time_zone_id` int unsigned NOT NULL AUTO_INCREMENT,
  `Use_leap_seconds` enum('Y','N') CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL DEFAULT 'N',
  PRIMARY KEY (`Time_zone_id`)
) /*!50100 TABLESPACE `mysql` */ ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 STATS_PERSISTENT=0 ROW_FORMAT=DYNAMIC COMMENT='Time zones'
1 row in set (0.01 sec)

`ENGINE=InnoDB` 表示该表使用的是 InnoDB 存储引擎。这是大多数配置中的默认存储引擎，因此你可能需要检查其状态。

你可以键入以下内容来显示 InnoDB 引擎的状态：

SHOW ENGINE INNODB STATUS\G

 *************************** 1. row ***************************
   Type: InnoDB
   Name:
 Status:
 =====================================
 2022-02-26 11:55:52 139992513984256 INNODB MONITOR OUTPUT
 =====================================
 Per second averages calculated from the last 16 seconds
 -----------------
 BACKGROUND THREAD
 -----------------
 srv_master_thread loops: 1 srv_active, 0 srv_shutdown, 1835091 srv_idle
 srv_master_thread log flush and writes: 0
 ----------
 SEMAPHORES
 ----------
 OS WAIT ARRAY INFO: reservation count 555
 OS WAIT ARRAY INFO: signal count 457
 RW-shared spins 0, rounds 0, OS waits 0
 RW-excl spins 0, rounds 0, OS waits 0
 RW-sx spins 0, rounds 0, OS waits 0
 Spin rounds per wait: 0.00 RW-shared, 0.00 RW-excl, 0.00 RW-sx
 ------------
 TRANSACTIONS
 ------------
 Trx id counter 8778
 Purge done for trx's n:o < 8713 undo n:o < 0 state: running but idle
 History list length 0
 LIST OF TRANSACTIONS FOR EACH SESSION:
 ---TRANSACTION 421467955133656, not started
 0 lock struct(s), heap size 1128, 0 row lock(s)
 ---TRANSACTION 421467955132848, not started
 0 lock struct(s), heap size 1128, 0 row lock(s)
 ---TRANSACTION 421467955132040, not started
 0 lock struct(s), heap size 1128, 0 row lock(s)
 --------
 FILE I/O
 --------
 I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
 I/O thread 1 state: waiting for completed aio requests (log thread)
 I/O thread 2 state: waiting for completed aio requests (read thread)
 I/O thread 3 state: waiting for completed aio requests (read thread)
 I/O thread 4 state: waiting for completed aio requests (read thread)
 I/O thread 5 state: waiting for completed aio requests (read thread)
 I/O thread 6 state: waiting for completed aio requests (write thread)
 I/O thread 7 state: waiting for completed aio requests (write thread)
 I/O thread 8 state: waiting for completed aio requests (write thread)
 I/O thread 9 state: waiting for completed aio requests (write thread)
 Pending normal aio reads: [0, 0, 0, 0] , aio writes: [0, 0, 0, 0] ,
  ibuf aio reads:, log i/o's:, sync i/o's:
 Pending flushes (fsync) log: 0; buffer pool: 0
 1779 OS file reads, 3384 OS file writes, 1870 OS fsyncs
 0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
 -------------------------------------
 INSERT BUFFER AND ADAPTIVE HASH INDEX
 -------------------------------------
 Ibuf: size 1, free list len 0, seg size 2, 0 merges
 merged operations:
  insert 0, delete mark 0, delete 0
 discarded operations:
  insert 0, delete mark 0, delete 0
 Hash table size 34679, node heap has 2 buffer(s)
 Hash table size 34679, node heap has 9 buffer(s)
 Hash table size 34679, node heap has 9 buffer(s)
 Hash table size 34679, node heap has 15 buffer(s)
 Hash table size 34679, node heap has 7 buffer(s)
 Hash table size 34679, node heap has 3 buffer(s)
 Hash table size 34679, node heap has 4 buffer(s)
 Hash table size 34679, node heap has 6 buffer(s)
 0.00 hash searches/s, 0.00 non-hash searches/s
 ---
 LOG
 ---
 Log sequence number          38540718
 Log buffer assigned up to    38540718
 Log buffer completed up to   38540718
 Log written up to            38540718
 Log flushed up to            38540718
 Added dirty pages up to      38540718
 Pages flushed up to          38540718
 Last checkpoint at           38540718
 2197 log i/o's done, 0.00 log i/o's/second
 ----------------------
 BUFFER POOL AND MEMORY
 ----------------------
 Total large memory allocated 0
 Dictionary memory allocated 473011
 Buffer pool size   8192
 Free buffers       6241
 Database pages     1896
 Old database pages 719
 Modified db pages  0
 Pending reads      0
 Pending writes: LRU 0, flush list 0, single page 0
 Pages made young 8, not young 0
 0.00 youngs/s, 0.00 non-youngs/s
 Pages read 1754, created 142, written 926
 0.00 reads/s, 0.00 creates/s, 0.00 writes/s
 No buffer pool page gets since the last printout
 Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
 LRU len: 1896, unzip_LRU len: 0
 I/O sum[0]:cur[0], unzip sum[0]:cur[0]
 --------------
 ROW OPERATIONS
 --------------
 0 queries inside InnoDB, 0 queries in queue
 0 read views open inside InnoDB
 Process ID=59529, Main thread ID=139992560166656 , state=sleeping
 Number of rows inserted 0, updated 0, deleted 0, read 0
 0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
 Number of system rows inserted 0, updated 317, deleted 0, read 6128
 0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
 ----------------------------
 END OF INNODB MONITOR OUTPUT
 ============================

 1 row in set (0.00 sec)

输出将包含大量有关引擎正在使用的资源、正在执行的进程等的详细信息。你可以使用它来了解执行过程中是否存在瓶颈，或者正在争用的进程数量是否导致性能问题。

检查 MySQL 是否正在记录慢速查询

你应该做的第一件事是验证慢速查询日志记录的当前状态。如果慢速查询日志记录已启用，则无需执行任何操作。

你可以键入以下内容来检查慢速查询日志记录是否已启用：

SHOW GLOBAL VARIABLES LIKE 'slow_query_log%';

 +---------------------+------------------------------------+
 | Variable_name       | Value                              |
 +---------------------+------------------------------------+
 | slow_query_log      | OFF                                |
 | slow_query_log_file | /var/lib/mysql/mysqlutils-slow.log |
 +---------------------+------------------------------------+
 2 rows in set (0.01 sec)

上面的输出表示当前未记录慢速查询，因为该功能已关闭。

如果慢速查询日志记录已打开，则你的输出将如下所示：

 +---------------------+-------------------------------+
 | Variable_name       | Value                         |
 +---------------------+-------------------------------+
 | slow_query_log      | ON                            |
 | slow_query_log_file | /var/log/mysql/mysql-slow.log |
 +---------------------+-------------------------------+
 2 rows in set (0.02 sec)

现在你知道了当前状态，可以根据需要进行更改。

配置 MySQL 记录慢查询

在我们继续之前，需要注意的是，尽管慢查询日志非常有用，但它可能会对性能产生额外的影响。MySQL 必须执行额外的操作来计时每个查询并记录结果到日志中。这会影响性能，并可能意外地填满硬盘空间。

可能并不总是需要记录慢查询。相反，在积极调查问题时启用该功能，并在完成后禁用它。

考虑到这一点，您可以通过修改 MySQL 服务器的配置文件来配置慢查询日志。您也可以交互式地修改这些值，但在配置文件中设置良好的默认值会使以后交互式调整变得更容易。

打开 MySQL 的配置文件。在大多数基于 Debian Linux 的系统上，配置文件位于 /etc/mysql/mysql.conf.d/mysqld.conf

sudo vim /etc/mysql/mysql.conf.d/mysqld.conf

我们需要修改或添加以下设置

例如，如果我们想打开所有可选日志记录，并记录任何至少检查 100 行且执行时间至少为 2 秒的语句，我们可以使用以下设置

; enable MySQL to log slow queries
slow_query_log                  = ON
; query log location
slow_query_log_file             = /var/log/mysql/mysql-slow.log
; amount of seconds that a query must surpass to be logged
long_query_time                 = 2
; minimum number of rows affected for a query to be logged
min_examined_row_limit          = 100

; include administrative commands in the logs
log_slow_admin_statements       = ON
; also log queries that are not using indexes
log_queries_not_using_indexes   = ON
; for MySQL servers version 8.0.14 or later, we can
; get extra information by setting this
log_slow_extra                  = ON
; Also log slow statements that have executed on
; the replica
; This only will log statements if `binlog_format` is
; set to `STATEMENT` or `MIXED` and the statement is
; logged in statement format
; RE: https://dev.mysqlserver.cn/doc/refman/8.0/en/replication-options-replica.html#sysvar_log_slow_replica_statements
; This option name depends on your MySQL version:
;   versions >= 8.0.26: log_slow_replica_statements
;   versions < 8.0.26: log_slow_slave_statements
; log_slow_slave_statements   = ON
log_slow_replica_statements   = ON

保存并关闭文件后，您可以通过键入以下命令来验证您的配置更改

sudo mysqld --validate-config

如果没有返回错误，则您的 MySQL 服务器配置文件语法有效。您可以通过键入以下命令重新启动 MySQL 服务器进程

sudo systemctl restart mysql

您可以通过重新运行原始的发现查询来验证慢查询是否已启用

SHOW GLOBAL VARIABLES LIKE 'slow_query_log%';

在配置好慢查询后，您可以在 MySQL 本身中根据需要启用和禁用它。调整值的语法如下

SET GLOBAL slow_query_log = 'OFF';