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(xún)事務(wù)隔離級(jí)別，默認(rèn)是RR
show variables like '%isolation%';

-- 設(shè)置事務(wù)隔離級(jí)別為RC
set session transaction isolation level read committed;


-- 數(shù)據(jù)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(xún)事務(wù)id
SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID();
-- INNODB 引擎狀態(tài)
SHOW ENGINE INNODB STATUS;
commit;

-- 二、可重復(fù)讀、不可重復(fù)讀示例

-- session1
set session transaction isolation level read committed;
begin;
-- session2
set session transaction isolation level repeatable read;
begin;
-- session1
select * from user;
-- session2
select * from user;
-- session3
begin;
insert into user (email, age, address) values ("test4@elsef.com", 30, "address4");
commit;
-- session1 這里因?yàn)槭荝C，所以可以讀到trx3提交的新數(shù)據(jù)，這里如果是證明不可重復(fù)讀的話應(yīng)該使用update而不是insert
select * from user;
commit;
-- session2 這里因?yàn)槭荝R，所以不會(huì)讀到trx3提交的新數(shù)據(jù)
select * from user;
commit;

-- 三、快照讀幻讀示例
-- session1
set session transaction isolation level repeatable read;
begin;
-- 這里使用快照讀
select * from user;
-- session2
begin;
insert into user (email, age, address) values ("test4@elsef.com", 30, "address4");
commit;
select * from user;
-- session1
select * from user; -- 這里讀不到test4@的數(shù)據(jù)，因?yàn)槭荝R
-- 這里發(fā)生了幻讀
insert into user (email, age, address) values ("test4@elsef.com", 30, "address4"); -- 插入失敗因?yàn)閑mail唯一索引沖突
commit;

-- 四、當(dāng)前讀幻讀示例
-- RC
-- session1
set session transaction isolation level read committed;
begin;
-- 這里會(huì)對(duì)所有滿(mǎn)足條件的age=20的記錄加鎖，因?yàn)槭荝C，所以沒(méi)有GAP鎖
delete from user where age = 20;
select * from user;
-- session2
set session transaction isolation level read committed;
begin;
-- 因?yàn)閠rx1沒(méi)有加GAP鎖，所以之類(lèi)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(shù)據(jù)，可以讀到trx1的刪除數(shù)據(jù)，因?yàn)槭荝C，trx1未提交所以沒(méi)影響trx2
commit;
-- session1
select * from user; -- 可以讀到trx2新插入的數(shù)據(jù)，雖然trx1是當(dāng)前讀，但是并未添加相應(yīng)的next-key鎖，沒(méi)有阻止trx2的新數(shù)據(jù)插入
commit;

--RR
-- session1
set session transaction isolation level repeatable read;
begin;
delete from user where age = 20;
select * from user;
-- session2
begin;
-- 這里會(huì)阻塞，因?yàn)閠rx1在age=20周?chē)恿薌AP鎖
-- 非唯一索引，首先，通過(guò)索引定位到第一條滿(mǎn)足查詢(xún)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(fù)進(jìn)行。直至進(jìn)行到第一條不滿(mǎn)足條件的記錄，此時(shí)，不需要加記錄X鎖，但是仍舊需要加GAP鎖，最后返回結(jié)束。
insert into user (email, age, address) values ("test4@elsef.com", 20, "address4");
-- 直到超時(shí)，ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
-- 此時(shí)如果查詢(xún)可以看到3條記錄
commit;
-- session1
-- 此時(shí)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
-- session1
set session transaction isolation level read committed;
begin;
delete from user where email = "test3@elsef.com";
-- session2
begin;
-- 可以讀到，因?yàn)閠rx1是RC
select * from user where email = "test3@elsef.com";
-- 嘗試更新這個(gè)記錄的age，會(huì)阻塞直到超時(shí)，因?yàn)閑mail是唯一索引已經(jīng)被trx1鎖住了，同時(shí)也會(huì)在對(duì)應(yīng)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;
-- session1
commit;
-- session2
commit;

-- 無(wú)索引+RC
-- session1
set session transaction isolation level read committed;
begin;
-- 由于address字段無(wú)索引，所以Innodb會(huì)對(duì)所有行進(jìn)行加鎖，由MySQL server進(jìn)行判斷并釋放鎖
delete from user where address = "address3";
-- session2
set session transaction isolation level read committed;
begin;
-- 這一行會(huì)成功，因?yàn)檫@一行沒(méi)有加鎖（先加了后釋放了）
update user set age = 10 where address = "address2";
-- 這一行同樣會(huì)被阻塞，原因是它已經(jīng)被trx1的語(yǔ)句加了鎖了，全部符合條件的都加鎖了
update user set age = 10 where address = "address3";
-- session1
commit;
-- session2
commit;

-- 非唯一索引+RR
-- session1
set session transaction isolation level repeatable read;
begin;
delete from user where age = 20;
-- session2
set session transaction isolation level repeatable read;
begin;
-- 這里會(huì)阻塞，因?yàn)閠rx1中已經(jīng)鎖住了age=20的記錄以及加上了GAP鎖，所以這里18已經(jīng)落入鎖區(qū)間
insert into user (email, age, address) values ("test4@elsef.com", 18, "address4");
-- session1
commit;
-- session2
commit;

-- 無(wú)索引RR
-- session1
set session transaction isolation level repeatable read;
begin;
-- 沒(méi)有索引，那么會(huì)鎖上表中的所有記錄，同時(shí)會(huì)鎖上主鍵索引上的所有GAP，杜絕所有的并發(fā)更新操作
delete from user where address = "address3";
-- session2
set session transaction isolation level repeatable read;
begin;
-- 這里會(huì)阻塞，原因是主鍵已經(jīng)被加上了GAP鎖，所以新的插入不能執(zhí)行成功
insert into user (email, age, address) values ("test4@elsef.com", 18, "address4");
-- session1
commit;
-- session2
commit;

-- 死鎖 簡(jiǎn)單示例
-- session1
begin;
delete from user where id = 1;
-- session2
begin;
delete from user where id = 3;
-- session1
delete from user where id = 3;
-- seession2
-- 這里MySQL判斷發(fā)生了死鎖，中斷了一個(gè)trx
-- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
delete from user where id = 1;
-- session1
rollback;
-- 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;
-- session1
begin;
insert into t1 values (2);
-- sessioin2
begin;
-- 這里會(huì)阻塞
insert into t1 values (2);
-- session3
begin;
-- 這里會(huì)阻塞
insert into t1 values (2);
-- session1;
-- 此時(shí)回滾，trx2和trx3收到通知，MySQL自動(dòng)中斷一個(gè)trx，因?yàn)榘l(fā)生了死鎖
-- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
rollback;
--session2;
rollback;
--session3;
rollback;