在MVCC中，讀操作可以分成兩類：快照讀 (snapshot read)與當前讀 (current read)。 快照讀，讀取的是記錄的可見版本 (有可能是歷史版本)，不用加鎖。當前讀，讀取的是記錄的最新版本，并且對返回的記錄，都會加上鎖，保證在事務結束前，這條數據都是最新版本。
-- 基本操作 --
-- 查詢事務隔離級別，默認是RR
show variables like '%isolation%';
-- 設置事務隔離級別為RC
set 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id
SELECT 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
-- 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insert
select * 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
-- 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
-- session1
set 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;
-- session2
set 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trx2
commit;
-- session1
select * 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
-- session1
set session transaction isolation level repeatable read;
begin;
delete from user where age = 20;
select * from user;
-- session2
begin;
-- 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
-- session1
set session transaction isolation level read committed;
begin;
delete from user where email = "test3@elsef.com";
-- session2
begin;
-- 可以讀到，因為trx1是RC
select * 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;
-- session1
commit;
-- session2
commit;
-- 無索引+RC
-- session1
set 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";
-- session2
set 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";
-- 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");
-- session1
commit;
-- session2
commit;
-- 無索引RR
-- session1
set 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";
-- session2
set 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");
-- session1
commit;
-- session2
commit;
-- 死鎖 簡單示例
-- 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判斷發生了死鎖，中斷了一個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;
-- 這里會阻塞
insert into t1 values (2);
-- session3
begin;
-- 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 transaction
rollback;
--session2;
rollback;
--session3;
rollback;
