鎖是在執行多線程時用於強行限定資源訪問的同步機制，數據庫鎖根據鎖的粒度可分爲行級鎖，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。行級鎖分爲共享鎖和排他鎖。
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，後者是通過在數據庫中對相應的數據行加鎖來實現的，InnoDB這種行級鎖決定，只有通過索引條件來檢索數據，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：
創建表結構
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;插入數據
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數據庫使用行鎖
打開三個命令行窗口進行測試
| 命令行窗口1 | 命令行窗口2 | 命令行窗口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）查詢非索引的字段來查詢數據庫使用行鎖
打開兩個命令行窗口進行測試
| 命令行窗口1 | 命令行窗口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）查詢非唯一索引字段來查詢數據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
打開三個命令行窗口進行測試
| 命令行窗口1 | 命令行窗口2 | 命令行窗口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數據庫時，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