select * from table where ?;

select * from table where ? lock in share mode;select * from table where ? for update;insert into table values ();update table set ? where ?;delete from table where ?;

-- 基本操作 ---- 查詢事務隔離級別，默認是RRshow variables like '%isolation%';-- 設置事務隔離級別為RCset session transaction isolation level read committed;-- 數據初始化 --begin;drop table if exists user;CREATE TABLE `user` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `email` varchar(64) NOT NULL, `age` int(11) NOT NULL, `address` varchar(64) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `uniq_email` (`email`), KEY `idx_age` (`age`));insert into user (email, age, address) values ("test1@elsef.com", 18, "address1");insert into user (email, age, address) values ("test2@elsef.com", 20, "address2");insert into user (email, age, address) values ("test3@elsef.com", 20, "address3");commit;select * from user;-- 一、trx_id示例begin;SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID();select * from user;SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID();SHOW ENGINE INNODB STATUS;update user set age = 22 where id = 3;-- 查詢事務idSELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID();-- INNODB 引擎狀態SHOW ENGINE INNODB STATUS;commit;-- 二、可重復讀、不可重復讀示例-- session1set session transaction isolation level read committed;begin;-- session2set session transaction isolation level repeatable read;begin;-- session1select * from user;-- session2select * from user;-- session3begin;insert into user (email, age, address) values ("test4@elsef.com", 30, "address4");commit;-- session1 這里因為是RC，所以可以讀到trx3提交的新數據，這里如果是證明不可重復讀的話應該使用update而不是insertselect * from user;commit;-- session2 這里因為是RR，所以不會讀到trx3提交的新數據select * from user;commit;-- 三、快照讀幻讀示例-- session1set session transaction isolation level repeatable read;begin;-- 這里使用快照讀select * from user;-- session2begin;insert into user (email, age, address) values ("test4@elsef.com", 30, "address4");commit;select * from user;-- session1select * from user; -- 這里讀不到test4@的數據，因為是RR-- 這里發生了幻讀insert into user (email, age, address) values ("test4@elsef.com", 30, "address4"); -- 插入失敗因為email唯一索引沖突commit;-- 四、當前讀幻讀示例-- RC-- session1set session transaction isolation level read committed;begin;-- 這里會對所有滿足條件的age=20的記錄加鎖，因為是RC，所以沒有GAP鎖delete from user where age = 20;select * from user;-- session2set session transaction isolation level read committed;begin;-- 因為trx1沒有加GAP鎖，所以之類可以插入age=20的記錄insert into user (email, age, address) values ("test4@elsef.com", 20, "address4");select * from user; -- 可以查到4條數據，可以讀到trx1的刪除數據，因為是RC，trx1未提交所以沒影響trx2commit;-- session1select * from user; -- 可以讀到trx2新插入的數據，雖然trx1是當前讀，但是并未添加相應的next-key鎖，沒有阻止trx2的新數據插入commit;--RR-- session1set session transaction isolation level repeatable read;begin;delete from user where age = 20;select * from user;-- session2begin;-- 這里會阻塞，因為trx1在age=20周圍加了GAP鎖-- 非唯一索引，首先，通過索引定位到第一條滿足查詢條件的記錄，加記錄上的X鎖，加GAP上的GAP鎖，然后加主鍵聚簇索引上的記錄X鎖；-- 然后讀取下一條，重復進行。直至進行到第一條不滿足條件的記錄，此時，不需要加記錄X鎖，但是仍舊需要加GAP鎖，最后返回結束。insert into user (email, age, address) values ("test4@elsef.com", 20, "address4");-- 直到超時，ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction-- 此時如果查詢可以看到3條記錄commit;-- session1-- 此時只能看到1條記錄，另外兩條被刪除了select * from user;commit;-- 唯一索引+RC-- session1set session transaction isolation level read committed;begin;delete from user where email = "test3@elsef.com";-- session2begin;-- 可以讀到，因為trx1是RCselect * from user where email = "test3@elsef.com";-- 嘗試更新這個記錄的age，會阻塞直到超時，因為email是唯一索引已經被trx1鎖住了，同時也會在對應的主鍵索引上加鎖-- 注意這里操作的id=3就是trx1中操作的email的同一行記錄update user set age = 40 where id = 3;-- session1commit;-- session2commit;-- 無索引+RC-- session1set session transaction isolation level read committed;begin;-- 由于address字段無索引，所以Innodb會對所有行進行加鎖，由MySQL server進行判斷并釋放鎖delete from user where address = "address3";-- session2set session transaction isolation level read committed;begin;-- 這一行會成功，因為這一行沒有加鎖（先加了后釋放了）update user set age = 10 where address = "address2";-- 這一行同樣會被阻塞，原因是它已經被trx1的語句加了鎖了，全部符合條件的都加鎖了update user set age = 10 where address = "address3";-- session1commit;-- session2commit;-- 非唯一索引+RR-- session1set session transaction isolation level repeatable read;begin;delete from user where age = 20;-- session2set session transaction isolation level repeatable read;begin;-- 這里會阻塞，因為trx1中已經鎖住了age=20的記錄以及加上了GAP鎖，所以這里18已經落入鎖區間insert into user (email, age, address) values ("test4@elsef.com", 18, "address4");-- session1commit;-- session2commit;-- 無索引RR-- session1set session transaction isolation level repeatable read;begin;-- 沒有索引，那么會鎖上表中的所有記錄，同時會鎖上主鍵索引上的所有GAP，杜絕所有的并發更新操作delete from user where address = "address3";-- session2set session transaction isolation level repeatable read;begin;-- 這里會阻塞，原因是主鍵已經被加上了GAP鎖，所以新的插入不能執行成功insert into user (email, age, address) values ("test4@elsef.com", 18, "address4");-- session1commit;-- session2commit;-- 死鎖 簡單示例-- session1begin;delete from user where id = 1;-- session2begin;delete from user where id = 3;-- session1delete from user where id = 3;-- seession2-- 這里MySQL判斷發生了死鎖，中斷了一個trx-- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transactiondelete from user where id = 1;-- session1rollback;-- session2;rollback;-- 五、死鎖 insert示例drop table if exists t1;begin;create table t1 ( `id` bigint not null auto_increment, primary key (`id`));insert into t1 values(1);insert into t1 values(5);commit;select * from t1;-- session1begin;insert into t1 values (2);-- sessioin2begin;-- 這里會阻塞insert into t1 values (2);-- session3begin;-- 這里會阻塞insert into t1 values (2);-- session1;-- 此時回滾，trx2和trx3收到通知，MySQL自動中斷一個trx，因為發生了死鎖-- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transactionrollback;--session2;rollback;--session3;rollback;