MySQL Lock--MySQL加锁学习1

准备测试数据：

## 开启InnoDB Monitor
SET GLOBAL innodb_status_output=ON;
SET GLOBAL innodb_status_output_locks=ON;

## 创建测试表
DROP TABLE IF EXISTS tb1001;

CREATE TABLE `tb1001` (
  `order_id` INT(11) NOT NULL,
  `order_num` INT(11) DEFAULT NULL,
  `order_type` INT(11) DEFAULT NULL,
  PRIMARY KEY (`order_id`),
  KEY `idx_order_type` (`order_type`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

## 准备测试数据
INSERT INTO tb1001(order_id,order_num,order_type)
VALUES(10,10,10),(20,10,20),(21,10,20),(30,10,30),(40,10,40);

## 查看当前表数据
SELECT * FROM tb1001;
+----------+-----------+------------+
| order_id | order_num | order_type |
+----------+-----------+------------+
|       10 |        10 |         10 |
|       20 |        10 |         20 |
|       21 |        10 |         20 |
|       30 |        10 |         30 |
|       40 |        10 |         40 |
+----------+-----------+------------+

测试1：

## 先执行事务A
BEGIN;
SELECT * FROM tb1001 WHERE order_id<=10 FOR UPDATE;

## 再执行事务B
## 事务B被阻塞
BEGIN;
INSERT INTO tb1001(order_id,order_num,order_type)
VALUES(19,20,10)

锁阻塞信息如下：

SELECT *
FROM `information_schema`.`INNODB_LOCKS` \G
*************************** 1. row ***************************
    lock_id: 1454153:29:3:3
lock_trx_id: 1454153
  lock_mode: X,GAP
  lock_type: RECORD
 lock_table: `db001`.`tb1001`
 lock_index: PRIMARY
 lock_space: 29
  lock_page: 3
   lock_rec: 3
  lock_data: 20
*************************** 2. row ***************************
    lock_id: 1454152:29:3:3
lock_trx_id: 1454152
  lock_mode: X
  lock_type: RECORD
 lock_table: `db001`.`tb1001`
 lock_index: PRIMARY
 lock_space: 29
  lock_page: 3
   lock_rec: 3
  lock_data: 20

SELECT *
FROM `information_schema`.`INNODB_LOCK_WAITS` \G
*************************** 1. row ***************************
requesting_trx_id: 1454153
requested_lock_id: 1454153:29:3:3
  blocking_trx_id: 1454152
 blocking_lock_id: 1454152:29:3:3

绿色部分表示申请锁成功，黄色部分表示申请锁被阻塞。

使用SHOW ENGINE INNODB STATUS 查看，输出锁信息为：

---TRANSACTION 1454152, ACTIVE 38 sec
## SQL:
## BEGIN;
## SELECT * FROM tb1001 WHERE order_id<=10 FOR UPDATE;

2 LOCK struct(s), HEAP size 1136, 2 ROW LOCK(s)
MySQL thread id 1475204, OS thread handle 139581472573184, QUERY id 4425986 127.0.0.1 admin
TABLE LOCK TABLE `db001`.`tb1001` trx id 1454152 LOCK MODE IX
RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454152 lock_mode X
Record LOCK, HEAP NO 2 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
 0: len 4; HEX 8000000a; ASC     ;; order_id=10
 1: len 6; HEX 00000016303e; ASC     0>;;
 2: len 7; HEX f6000000320110; ASC     2  ;;
 3: len 4; HEX 8000000a; ASC     ;;
 4: len 4; HEX 8000000a; ASC     ;;

Record LOCK, HEAP NO 3 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
 0: len 4; HEX 80000014; ASC     ;; order_id=20
 1: len 6; HEX 00000016303e; ASC     0>;;
 2: len 7; HEX f600000032011c; ASC     2  ;;
 3: len 4; HEX 8000000a; ASC     ;;
 4: len 4; HEX 80000014; ASC     ;;

---TRANSACTION 1454153, ACTIVE 17 sec inserting
## SQL:
## BEGIN;
## INSERT INTO tb1001(order_id,order_num,order_type)
## VALUES(19,20,10);

mysql TABLES IN USE 1, locked 1
LOCK WAIT 2 LOCK struct(s), HEAP size 1136, 1 ROW LOCK(s)
MySQL thread id 1475203, OS thread handle 139581473105664, QUERY id 4425988 127.0.0.1 admin UPDATE
INSERT INTO tb1001(order_id,order_num,order_type)
VALUES(19,20,10)

------- TRX HAS BEEN WAITING 17 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454153 lock_mode X LOCKS gap BEFORE rec INSERT intention waiting
Record LOCK, HEAP NO 3 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
 0: len 4; HEX 80000014; ASC     ;;  order_id=20
 1: len 6; HEX 00000016303e; ASC     0>;;
 2: len 7; HEX f600000032011c; ASC     2  ;;
 3: len 4; HEX 8000000a; ASC     ;;
 4: len 4; HEX 80000014; ASC     ;;

------------------
TABLE LOCK TABLE `db001`.`tb1001` trx id 1454153 LOCK MODE IX
RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454153 lock_mode X LOCKS gap BEFORE rec INSERT intention waiting
Record LOCK, HEAP NO 3 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
 0: len 4; HEX 80000014; ASC     ;; order_id=20
 1: len 6; HEX 00000016303e; ASC     0>;;
 2: len 7; HEX f600000032011c; ASC     2  ;;
 3: len 4; HEX 8000000a; ASC     ;;
 4: len 4; HEX 80000014; ASC     ;;

加锁详解：

## SQL
## BEGIN;
## SELECT * FROM tb1001 WHERE order_id<=10 FOR UPDATE;

在可重复读事务隔离级别下,由于 WHERE order_id<=10 需要对主键做范围扫描，使用加锁规则：
1、对满足条件的记录加Next-key锁。
2、从左向右扫描满足条件的记录，当遇到第一条不满足条件记录时，对该记录和该记录之前的间隙加锁(NEXT-KEY LOCK)。

加锁操作如下
1、当扫描到记录order_id=10时，满足条件，对记录order_id=10加Next-key锁,锁信息为：
    RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454152 lock_mode X
    Record LOCK, HEAP NO 2 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
    0: len 4; HEX 8000000a; ASC     ;; order_id=10
2、继续扫描到记录order_id=20时，第一条不满足条件的，对记录order_id=20加Next-key锁，锁信息为：
    RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454152 lock_mode X
    Record LOCK, HEAP NO 3 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
    0: len 4; HEX 80000014; ASC     ;; order_id=20

## SQL
## BEGIN;
## insert into tb1001(order_id,order_num,order_type)
## values(19,20,10)

INSERT加锁规则如下：
1、INSERT操作会对新插入的记录加行锁(ROW LOCK)+排他锁(X LOCK),不会产生任何GAP锁和Next-Key锁
2、在插入记录前，会向插入记录所在位置申请意向插入Gap锁(Insertion Intention Gap LOCK),相同区间的意向插入Gap锁不会冲突。
3、对于唯一索引，如果插入记录时表中已存在相同键值记录(被其他事务修改且未提交)，即存在唯一键冲突，会尝试在已有记录上加读锁，然后等待。

加锁操作如下：
1、对表tb1001做数据插入操作，需要对表tb001上申请意向锁(TABLE LOCK TABLE `db001`.`tb1001` trx id 1454153 LOCK MODE IX)
2、向新记录order_id=19所在位置申请插入Gap锁(Insertion Intention Gap LOCK)，但由于上面事务对记录order_id=20加Next-key锁，申请失败处于等待状态(lock_mode X LOCKS gap BEFORE rec INSERT intention waiting),等待锁信息为：

　　Record LOCK, HEAP NO 3 PHYSICAL RECORD: n_fields 5; COMPACT FORMAT; info bits 0
　　0: len 4; HEX 80000014; ASC ;; order_id=20

GAP LOCK /RECORD LOCK /NEXT KEY LOCK

在输出的锁信息中，如果仅对记录加行锁且未对记录前空间加GAP锁，则锁信息为：

RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454154 lock_mode X LOCKS rec but NOT gap

在输出的锁信息中，如果记录加行锁且对记录前空间加GAP锁，则锁信息为：

RECORD LOCKS SPACE id 29 page NO 3 n bits 80 INDEX PRIMARY of TABLE `db001`.`tb1001` trx id 1454152 lock_mode X

未显式指明"but NOT GAP"的RECORD LOCK实际上NEXT KEY LOCK。

在输出的锁信息中，如果未对记录加锁且仅对记录前空间加GAP锁，则锁信息为：

RECORD LOCKS space id 29 page no 3 n bits 80 index PRIMARY of table `db001`.`tb1001` trx id 1454596 lock_mode X locks gap before rec