識別 MySQL 中的慢查詢

顯示完整程序列表

要顯示 MySQL 處理執行緒正在執行的所有當前操作，請輸入

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)

以上輸出顯示了一個空閒伺服器，其中只有我們自己的查詢以及一個長時間執行的事件監聽器。一個活躍的伺服器會顯示更多的程序，其中一些可能是長時間執行的。如果沒有 FULL 修飾符，此命令將只顯示前 100 個程序，這可能會或可能不會根據您的伺服器活動截斷結果。

需要檢視的一些重要部分是 Time 和 State 列。Time 列計算執行緒處於所述 State 的秒數。如果您發現某些程序的 Time 值與您對給定操作的預期不符，那麼可能是時候進一步調查了。

檢查儲存引擎狀態

另一個可以檢查的地方是實際儲存引擎的狀態。

您可以透過輸入以下命令找到與給定表關聯的儲存引擎：

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

例如，要顯示 mysql.time_zone 表使用的儲存引擎，請輸入

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 表示該表正在使用 InnoDB 儲存引擎。這是大多數配置中的預設儲存引擎，因此您可能需要檢查其狀態。

您可以透過輸入以下命令顯示 InnoDB 引擎的狀態：

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)

輸出將包含大量資訊，包括引擎正在使用的資源、正在執行的程序等。您可以使用它來了解執行是否存在瓶頸，或者競爭的程序數量是否導致效能問題。

檢查 MySQL 是否正在記錄慢查詢

您首先應該做的是驗證慢查詢日誌的當前狀態。如果慢查詢日誌已啟用，您無需執行任何操作。

您可以透過輸入以下命令檢查慢查詢日誌是否已啟用：

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)

以上輸出表明慢查詢目前未被記錄，因為該功能已關閉。

如果慢查詢日誌已開啟，您的輸出將看起來像這樣：

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

現在您知道了當前狀態，可以根據需要進行更改。

配置 MySQL 以記錄慢查詢

在我們繼續之前，重要的是要注意，雖然慢查詢日誌非常有用，但它可能會對效能產生額外的開銷。MySQL 必須執行額外的操作來計時每個查詢並將結果記錄到日誌中。這可能會影響效能並意外地佔用硬碟空間。

不建議始終記錄慢查詢。相反，在您積極調查問題時啟用此功能，並在完成後停用它。

考慮到這一點，您可以透過修改 MySQL 伺服器的配置檔案來配置慢查詢日誌。您也可以互動式地修改這些值，但在配置中設定良好的預設值將使以後互動式調整變得更容易。

開啟 MySQL 的配置檔案。在大多數基於 Debian Linux 的系統上，配置檔案位於 /etc/mysql/mysql.conf.d/mysqld.conf

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

我們將需要修改或可能新增以下設定：

因此，例如，如果我們想開啟所有可選日誌記錄，並記錄任何檢查至少 100 行且執行時間超過 2 秒的語句，我們可以使用這些設定：

; 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.mysql.com.tw/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

儲存並關閉檔案後，您可以透過輸入以下命令來驗證您的配置更改：

sudo mysqld --validate-config

如果沒有返回錯誤，則您的 MySQL 伺服器配置檔案在語法上是有效的。您可以透過輸入以下命令來重新啟動 MySQL 伺服器程序：

sudo systemctl restart mysql

您現在可以透過重新執行原始發現查詢來驗證慢查詢是否已啟用。

SHOW GLOBAL VARIABLES LIKE 'slow_query_log%';

一旦您按照自己的意願配置了慢查詢，就可以在 MySQL 內部根據需要啟用和停用它。調整值的語法如下所示：

SET GLOBAL slow_query_log = 'OFF';