前言
鎖是在執(zhí)行多線程時(shí)用于強(qiáng)行限定資源訪問(wèn)的同步機(jī)制,數(shù)據(jù)庫(kù)鎖根據(jù)鎖的粒度可分為行級(jí)鎖,表級(jí)鎖和頁(yè)級(jí)鎖
行級(jí)鎖
行級(jí)鎖是mysql中粒度最細(xì)的一種鎖機(jī)制,表示只對(duì)當(dāng)前所操作的行進(jìn)行加鎖,行級(jí)鎖發(fā)生沖突的概率很低,其粒度最小,但是加鎖的代價(jià)最大。行級(jí)鎖分為共享鎖和排他鎖。
特點(diǎn):
開(kāi)銷大,加鎖慢,會(huì)出現(xiàn)死鎖;鎖定粒度最小,發(fā)生鎖沖突的概率最大,并發(fā)性也高;
實(shí)現(xiàn)原理:
InnoDB行鎖是通過(guò)給索引項(xiàng)加鎖來(lái)實(shí)現(xiàn)的,這一點(diǎn)mysql和oracle不同,后者是通過(guò)在數(shù)據(jù)庫(kù)中對(duì)相應(yīng)的數(shù)據(jù)行加鎖來(lái)實(shí)現(xiàn)的,InnoDB這種行級(jí)鎖決定,只有通過(guò)索引條件來(lái)檢索數(shù)據(jù),才能使用行級(jí)鎖,否則,直接使用表級(jí)鎖。特別注意:使用行級(jí)鎖一定要使用索引
舉個(gè)栗子:
創(chuàng)建表結(jié)構(gòu)
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(shù)據(jù)
INSERT INTO `developerinfo` VALUES ('1', 'liujie', '123456'); INSERT INTO `developerinfo` VALUES ('2', 'yitong', '123'); INSERT INTO `developerinfo` VALUES ('3', 'tong', '123456');
(1)通過(guò)主鍵索引來(lái)查詢數(shù)據(jù)庫(kù)使用行鎖
打開(kāi)三個(gè)命令行窗口進(jìn)行測(cè)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(lái)查詢數(shù)據(jù)庫(kù)使用行鎖
打開(kāi)兩個(gè)命令行窗口進(jìn)行測(cè)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(lái)查詢數(shù)據(jù)庫(kù)使用行鎖鎖住多行
mysql的行鎖是針對(duì)索引假的鎖,不是針對(duì)記錄,所以可能會(huì)出現(xiàn)鎖住不同記錄的場(chǎng)景
打開(kāi)三個(gè)命令行窗口進(jìn)行測(cè)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(lái)操作檢索數(shù)據(jù)庫(kù)時(shí),是否使用索引還需有mysql通過(guò)判斷不同執(zhí)行計(jì)劃來(lái)決定,是否使用該索引,如需判定如何使用explain來(lái)判斷索引,請(qǐng)聽(tīng)下回分解
總結(jié)
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標(biāo)簽:鄂爾多斯 黔西 昌都 駐馬店 荊門(mén) 梅河口 陜西 北京
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