會(huì)話事務(wù)中查找的數(shù)據(jù)，加上一個(gè)共享鎖。若會(huì)話事務(wù)中查找的數(shù)據(jù)已經(jīng)被其他會(huì)話事務(wù)加上獨(dú)占鎖的話，共享鎖會(huì)等待其結(jié)束再加，若等待時(shí)間過長就會(huì)顯示事務(wù)需要的鎖等待超時(shí)。

2.2 SELECT ….. FOR UPDATE

會(huì)話事務(wù)中查找的數(shù)據(jù)，加上一個(gè)讀更新瑣，其他會(huì)話事務(wù)將無法再加其他鎖，必須等待其結(jié)束。

2.3 INSERT、UPDATE、DELETE

會(huì)話事務(wù)會(huì)對(duì)DML語句操作的數(shù)據(jù)加上一個(gè)獨(dú)占鎖，其他會(huì)話的事務(wù)都將會(huì)等待其釋放獨(dú)占鎖。

2.4 gap and next key lock（間隙鎖）

InnoDB引擎會(huì)自動(dòng)給會(huì)話事務(wù)中的共享鎖、更新瑣以及獨(dú)占鎖，需要加到一個(gè)區(qū)間值域的時(shí)候，再加上個(gè)間隙鎖（或稱范圍鎖），對(duì)不存在的數(shù)據(jù)也鎖住，防止出現(xiàn)幻寫。

備注：

以上2.1，2.2，2.3，2.4中描述的情況，跟MySQL所設(shè)置的事務(wù)隔離級(jí)別也有關(guān)系。

3.四種事務(wù)隔離模式

3.1 READ UNCOMMITED

SELECT的時(shí)候允許臟讀，即SELECT會(huì)讀取其他事務(wù)修改而還沒有提交的數(shù)據(jù)。

3.2 READ COMMITED

SELECT的時(shí)候無法重復(fù)讀，即同一個(gè)事務(wù)中兩次執(zhí)行同樣的查詢語句，若在第一次與第二次查詢之間時(shí)間段，其他事務(wù)又剛好修改了其查詢的數(shù)據(jù)且提交了，則兩次讀到的數(shù)據(jù)不一致。

3.3 REPEATABLE READ

SELECT的時(shí)候可以重復(fù)讀，即同一個(gè)事務(wù)中兩次執(zhí)行同樣的查詢語句，得到的數(shù)據(jù)始終都是一致的。

3.4 SERIALIZABLE

與可重復(fù)讀的唯一區(qū)別是，默認(rèn)把普通的SELECT語句改成SELECT …. LOCK IN SHARE MODE。即為查詢語句涉及到的數(shù)據(jù)加上共享瑣，阻塞其他事務(wù)修改真實(shí)數(shù)據(jù)。

4. 驗(yàn)證事務(wù)與鎖定示例

接下來，我們將以MySQL中的InnoDB引擎，解釋其如何實(shí)現(xiàn)ACID特性，不同隔離級(jí)別下事務(wù)與事務(wù)之間的影響。示例表結(jié)構(gòu)：

CREATE TABLE `account ` (

`ID` int(11) NOT NULL AUTO_INCREMENT,

`VACCOUNT_ID` varchar(32) NOT NULL,

`GMT_CREATE` datetime NOT NULL,

PRIMARY KEY (`ID`),

KEY `idx_VACCOUNT_PARAMETER_VACCOUNTID ` (`VACCOUNT_ID`)

) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE utf8_general_ci;

然后向表account中寫入10W條創(chuàng)建日期分布合理的帳號(hào)數(shù)據(jù)，以方便測(cè)試之用。

tx_isolation：SET GLOBAL tx_isolation='read-uncommitted'
ID	事務(wù)1	事務(wù)1輸出	事務(wù)2	事務(wù)2輸出
1	START TRANSACTION;
2	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
			START TRANSACTION;
3			UPDATE account set VACCOUNT_ID='uncommitted' where ID =1001;
4			SELECT VACCOUNT_ID from account where ID =1001;	uncommitted
5	SELECT VACCOUNT_ID from account where ID =1001;	uncommitted
6			ROLLBACK;
7	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
8	COMMIT;

tx_isolation：SET GLOBAL tx_isolation='read-committed'
ID	事務(wù)1	事務(wù)1輸出	事務(wù)2	事務(wù)2輸出
1	START TRANSACTION;
2	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
3			START TRANSACTION;
4			UPDATE account set VACCOUNT_ID='uncommitted' where ID =1001;
5			SELECT VACCOUNT_ID from account where ID =1001;	uncommitted
6	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
7			COMMIT;
8	SELECT VACCOUNT_ID from account where ID =1001;	uncommitted
9	COMMIT;

tx_isolation：SET GLOBAL tx_isolation='REPEATABLE-READ'
ID	事務(wù)1	事務(wù)1輸出	事務(wù)2	事務(wù)2輸出
1	START TRANSACTION;
2	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
3			START TRANSACTION;
4			UPDATE account set VACCOUNT_ID='uncommitted' where ID =1001;
5			SELECT VACCOUNT_ID from account where ID =1001;	uncommitted
6	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
7			COMMIT;
8	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
9	COMMIT;

tx_isolation：SET GLOBAL tx_isolation='SERIALIZABLE'
ID	事務(wù)1	事務(wù)1輸出	事務(wù)2	事務(wù)2輸出
1	START TRANSACTION;
2	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
3			START TRANSACTION;
4			UPDATE account set VACCOUNT_ID='uncommitted' where ID =1001;	STATE: Updating
5	SELECT VACCOUNT_ID from account where ID =1001;	caimao101510
			事務(wù)2超時(shí)
6	COMMIT;
7	START TRANSACTION;
8	UPDATE account set VACCOUNT_ID='uncommitted' where ID =1001;
9			START TRANSACTION;
10			SELECT VACCOUNT_ID from account where ID =1001;	STATE：statistics
11			事務(wù)2超時(shí)
12	commit;

tx_isolation：SET GLOBAL tx_isolation='REPEATABLE-READ'
ID	事務(wù)1	事務(wù)1輸出	事務(wù)2	事務(wù)2輸出
1	START TRANSACTION;
2	select max(ID) FROM account;	124999
3			START TRANSACTION;
4	UPDATE account set gmt_create=date_add(gmt_create,interval +1 day) WHERE ID >=124999;
5			insert into account(VACCOUNT_ID,gmt_create) values(‘eugene',now());	STATE:update
6			事務(wù)2超時(shí)
7			START TRANSACTION;
8			SELECT * FROM account WHERE ID =124998;	2007-10-20 13:47
9			UPDATE account set gmt_create=date_add(gmt_create,interval +1 day) WHERE ID =124998;	執(zhí)行成功
10			SELECT * FROM account WHERE ID =124998;	2007-10-21 13:47
11	COMMIT;
12			COMMIT;

1	START TRANSACTION;
2	UPDATE account set gmt_create=date_add(gmt_create,interval -1 day) WHERE gmt_create >'2009-07-01′;
3			START TRANSACTION;
4			SELECT * FROM account WHERE gmt_create>'2009-07-10′ LIMIT 1;	2009-10-2 13:47
5	SELECT * FROM account WHERE gmt_create>'2009-07-10′ LIMIT 1;	2009-10-1 13:47		STATE:update
6			insert into account(VACCOUNT_ID,gmt_create) values(‘gmt_create_test',now());
7			事務(wù)2超時(shí)
8	COMMIT;
9			SELECT * FROM account WHERE gmt_create>'2009-07-10′ LIMIT 1;	2009-10-1 13:47
無索引條件更新事務(wù)
1	START TRANSACTION;
	UPDATE account set gmt_create=date_add(gmt_create,interval -1 day) WHERE gmt_create >'2009-07-01′ AND gmt_create <'2009-07-10′;
			START TRANSACTION;
			insert into account(VACCOUNT_ID,gmt_create) values(‘gmt_create_interval',now());
			事務(wù)2超時(shí)
	COMMIT;