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前言
MySQL數據庫鎖是實現數據一致性，解決并發問題的重要手段。數據庫是一個多用戶共享的資源，當出現并發的時候，就會導致出現各種各樣奇怪的問題，就像程序代碼一樣，出現多線程并發的時候，如果不做特殊控制的話，就會出現意外的事情，比如“臟“數據、修改丟失等問題。所以數據庫并發需要使用事務來控制，事務并發問題需要數據庫鎖來控制，所以數據庫鎖是跟并發控制和事務聯系在一起的。
本文主要描述基于更新SQL語句來理解MySQL鎖定。下面話不多說了，來一起看看詳細的介紹吧
一、構造環境
(root@localhost) [user]> show variables like 'version'; +---------------+------------+ | Variable_name | Value | +---------------+------------+ | version | 5.7.23-log | +---------------+------------+ (root@localhost) [user]> desc t1; +-------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | n | int(11) | YES | | NULL | | | table_name | varchar(64) | YES | | NULL | | | column_name | varchar(64) | YES | | NULL | | | pad | varchar(100) | YES | | NULL | | +-------------+--------------+------+-----+---------+----------------+ (root@localhost) [user]> select count(*) from t1; +----------+ | count(*) | +----------+ | 3406 | +----------+ (root@localhost) [user]> create unique index idx_t1_pad on t1(pad); Query OK, 0 rows affected (0.35 sec) Records: 0 Duplicates: 0 Warnings: 0 (root@localhost) [user]> create index idx_t1_n on t1(n); Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 (root@localhost) [user]> show index from t1; +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Null | Index_type | +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+ | t1 | 0 | PRIMARY | 1 | id | A | 3462 | | BTREE | | t1 | 0 | idx_t1_pad | 1 | pad | A | 3406 | YES | BTREE | | t1 | 1 | idx_t1_n | 1 | n | A | 12 | YES | BTREE | +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+ select 'Leshami' author,'http://blog.csdn.net/leshami' Blog; +---------+------------------------------+ | author | Blog | +---------+------------------------------+ | Leshami | http://blog.csdn.net/leshami | +---------+------------------------------+
二、基于主鍵更新
(root@localhost) [user]> start transaction; Query OK, 0 rows affected (0.00 sec) (root@localhost) [user]> update t1 set table_name='t1' where id=1299; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 SELECT trx_id, trx_state, trx_started, trx_mysql_thread_id, trx_tables_locked, trx_rows_locked, trx_rows_modified, trx_isolation_level FROM INFORMATION_SCHEMA.INNODB_TRX \G -- 從下面的結果可知，trx_rows_locked，一行被鎖定 *************************** 1. row *************************** trx_id: 6349647 trx_state: RUNNING trx_started: 2018-11-06 16:54:12 trx_mysql_thread_id: 2 trx_tables_locked: 1 trx_rows_locked: 1 trx_rows_modified: 1 trx_isolation_level: REPEATABLE READ (root@localhost) [user]> rollback; Query OK, 0 rows affected (0.01 sec)
三、基于二級唯一索引
(root@localhost) [user]> start transaction; Query OK, 0 rows affected (0.00 sec) (root@localhost) [user]> update t1 set table_name='t2' where pad='4f39e2a03df3ab94b9f6a48c4aecdc0b'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 SELECT trx_id, trx_state, trx_started, trx_mysql_thread_id, trx_tables_locked, trx_rows_locked, trx_rows_modified, trx_isolation_level FROM INFORMATION_SCHEMA.INNODB_TRX \G -- 從下面的查詢結果可知，trx_rows_locked，2行被鎖定 *************************** 1. row *************************** trx_id: 6349649 trx_state: RUNNING trx_started: 2018-11-06 16:55:22 trx_mysql_thread_id: 2 trx_tables_locked: 1 trx_rows_locked: 2 trx_rows_modified: 1 trx_isolation_level: REPEATABLE READ (root@localhost) [user]> rollback; Query OK, 0 rows affected (0.00 sec)
三、基于二級非唯一索引
(root@localhost) [user]> start transaction; Query OK, 0 rows affected (0.00 sec) (root@localhost) [user]> update t1 set table_name='t3' where n=8; Query OK, 350 rows affected (0.01 sec) Rows matched: 351 Changed: 351 Warnings: 0 SELECT trx_id, trx_state, trx_started, trx_mysql_thread_id, trx_tables_locked, trx_rows_locked, trx_rows_modified, trx_isolation_level FROM INFORMATION_SCHEMA.INNODB_TRX \G --從下面的查詢結果可知，703行被鎖定 *************************** 1. row *************************** trx_id: 6349672 trx_state: RUNNING trx_started: 2018-11-06 17:06:53 trx_mysql_thread_id: 2 trx_tables_locked: 1 trx_rows_locked: 703 trx_rows_modified: 351 trx_isolation_level: REPEATABLE READ (root@localhost) [user]> rollback; Query OK, 0 rows affected (0.00 sec)
四、無索引更新
(root@localhost) [user]> start transaction; Query OK, 0 rows affected (0.00 sec) (root@localhost) [user]> update t1 set table_name='t4' where column_name='id'; Query OK, 26 rows affected (0.00 sec) Rows matched: 26 Changed: 26 Warnings: 0 SELECT trx_id, trx_state, trx_started, trx_mysql_thread_id, trx_tables_locked, trx_rows_locked, trx_rows_modified, trx_isolation_level FROM INFORMATION_SCHEMA.INNODB_TRX \G -- 從下面的查詢結果可知，trx_rows_locked，3429行被鎖定，而被更新的僅僅為26行 -- 而且這個結果超出了表上的總行數3406 *************************** 1. row *************************** trx_id: 6349674 trx_state: RUNNING trx_started: 2018-11-06 17:09:41 trx_mysql_thread_id: 2 trx_tables_locked: 1 trx_rows_locked: 3429 trx_rows_modified: 26 trx_isolation_level: REPEATABLE READ (root@localhost) [user]> rollback; Query OK, 0 rows affected (0.00 sec) -- 也可以通過show engine innodb status進行觀察 show engine innodb status\G ------------ TRANSACTIONS ------------ Trx id counter 6349584 Purge done for trx's n:o < 0 undo n:o < 0 state: running but idle History list length 0 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 421943222819552, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 6349583, ACTIVE 2 sec 2 lock struct(s), heap size 1136, 1 row lock(s), undo log entries 1 ------------ TRANSACTIONS ------------ Trx id counter 6349586 Purge done for trx's n:o < 6349585 undo n:o < 0 state: running but idle History list length 1 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 421943222819552, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 6349585, ACTIVE 8 sec 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1 MySQL thread id 2, OS thread handle 140467640694528, query id 29 localhost root
五、鎖相關查詢SQL
1：查看當前的事務
SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX;
2：查看當前鎖定的事務
SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCKS;
3：查看當前等鎖的事務
SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCK_WAITS; SELECT trx_id, trx_state, trx_started, trx_mysql_thread_id thr_id, trx_tables_locked tb_lck, trx_rows_locked rows_lck, trx_rows_modified row_mfy, trx_isolation_level is_lvl FROM INFORMATION_SCHEMA.INNODB_TRX; SELECT r.`trx_id` waiting_trx_id, r.`trx_mysql_thread_id` waiting_thread, r.`trx_query` waiting_query, b.`trx_id` bolcking_trx_id, b.`trx_mysql_thread_id` blocking_thread, b.`trx_query` block_query FROM information_schema.`INNODB_LOCK_WAITS` w INNER JOIN information_schema.`INNODB_TRX` b ON b.`trx_id` = w.`blocking_trx_id` INNER JOIN information_schema.`INNODB_TRX` r ON r.`trx_id` = w.`requesting_trx_id`;
六、小結
1、MySQL表更新時，對記錄的鎖定根據更新時where謂詞條件來確定鎖定范圍
2、對于聚簇索引過濾，由于索引即數據，因為僅僅鎖定更新行，這是由聚簇索引的性質決定的
3、對于非聚簇唯一索引過濾，由于需要回表，因此鎖定為唯一索引過濾行數加上回表行數
4、對于非聚簇非唯一索引過濾，涉及到了間隙鎖，因此鎖定的記錄數更多
5、如果過濾條件無任何索引或無法使用到索引，則鎖定整張表上所有數據行
總結
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