前言

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

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