mysql鎖機制之行級鎖
阿新 • • 發佈:2019-02-11
鎖是在執行多執行緒時用於強行限定資源訪問的同步機制,資料庫鎖根據鎖的粒度可分為行級鎖,表級鎖和頁級鎖
行級鎖
行級鎖是mysql中粒度最細的一種鎖機制,表示只對當前所操作的行進行加鎖,行級鎖發生衝突的概率很低,其粒度最小,但是加鎖的代價最大。行級鎖分為共享鎖和排他鎖。
特點:
開銷大,加鎖慢,會出現死鎖;鎖定粒度最小,發生鎖衝突的概率最大,併發性也高;
實現原理:
InnoDB行鎖是通過給索引項加鎖來實現的,這一點mysql和oracle不同,後者是通過在資料庫中對相應的資料行加鎖來實現的,InnoDB這種行級鎖決定,只有通過索引條件來檢索資料,才能使用行級鎖,否則,直接使用表級鎖。特別注意:使用行級鎖一定要使用索引
舉個栗子:
建立表結構
CREATE TABLE `developerinfo` (
`userID` bigint(20) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`passWord` varchar(255) DEFAULT NULL,
PRIMARY KEY (`userID`),
KEY `PASSWORD_INDEX` (`passWord`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
插入資料
INSERT INTO `developerinfo` VALUES ('1', 'liujie', '123456'); INSERT INTO `developerinfo` VALUES ('2', 'yitong', '123'); INSERT INTO `developerinfo` VALUES ('3', 'tong', '123456');
(1)通過主鍵索引來查詢資料庫使用行鎖
開啟三個命令列視窗進行測試
命令列視窗1 | 命令列視窗2 | 命令列視窗3 |
mysql> set autocommit = 0; Query OK, 0 rows affected mysql> select * from developerinfo where userid = '1' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | +--------+--------+----------+ 1 row in set | mysql> set autocommit = 0; Query OK, 0 rows affected mysql> select * from developerinfo where userid = '1' for update; 等待 | mysql> set autocommit = 0; Query OK, 0 rows affected mysql> select * from developerinfo where userid = '3' for update; +--------+------+----------+ | userID | name | passWord | +--------+------+----------+ | 3 | tong | 123456 | +--------+------+----------+ 1 row in set |
mysql> commit; Query OK, 0 rows affected | mysql> select * from developerinfo where userid = '1' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | +--------+--------+----------+ 1 row in set |
(2)查詢非索引的欄位來查詢資料庫使用行鎖
開啟兩個命令列視窗進行測試
命令列視窗1 | 命令列視窗2 |
mysql> set autocommit=0; Query OK, 0 rows affected mysql> select * from developerinfo where name = 'liujie' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | +--------+--------+----------+ 1 row in set | mysql> set autocommit=0; Query OK, 0 rows affected mysql> select * from developerinfo where name = 'tong' for update; 等待 |
mysql> commit; Query OK, 0 rows affected | mysql> select * from developerinfo where name = 'liujie' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | +--------+--------+----------+ 1 row in set |
(3)查詢非唯一索引欄位來查詢資料庫使用行鎖鎖住多行
mysql的行鎖是針對索引假的鎖,不是針對記錄,所以可能會出現鎖住不同記錄的場景
開啟三個命令列視窗進行測試
命令列視窗1 | 命令列視窗2 | 命令列視窗3 |
mysql> set autocommit=0; Query OK, 0 rows affected mysql> select * from developerinfo where password = '123456 ' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | | 3 | tong | 123456 | +--------+--------+----------+ 2 rows in set | mysql> set autocommit =0 ; Query OK, 0 rows affected mysql> select * from developerinfo where userid = '1' for update; 等待 | mysql> set autocommit = 0; Query OK, 0 rows affected mysql> select * from developerinfo where userid = '2 ' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 2 | yitong | 123 | +--------+--------+----------+ 1 row in set |
commit; | mysql> select * from developerinfo where userid = '1' for update; +--------+--------+----------+ | userID | name | passWord | +--------+--------+----------+ | 1 | liujie | 123456 | +--------+--------+----------+ 1 row in set |
(4)條件中使用索引來操作檢索資料庫時,是否使用索引還需有mysql通過判斷不同執行計劃來決定,是否使用該索引,如需判定如何使用explain來判斷索引,請聽下回分解