MySQL Lock--gap before rec insert intention waiting

在事务插入数据过程中，为防止其他事务向索引上该位置插入数据，会在插入之前先申请插入意向范围锁，而如果申请插入意向范围锁被阻塞，则事务处于gap before rec insert intention waiting的等待状态。

MySQL官方文档解释如下：

INSERT sets an exclusive lock on the inserted row. This lock is an index-record lock, not a next-key lock (that is, there is no gap lock) and does not prevent other sessions from inserting into the gap before the inserted row.Prior to inserting the row, a type of gap lock called an insertion intention gap lock is set. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap.If a duplicate-key error occurs, a shared lock on the duplicate index record is set. This use of a shared lock can result in deadlock should there be multiple sessions trying to insert the same row if another session already has an exclusive lock.

准备测试数据：

## 创建测试表
CREATE TABLE `tb1001` (
  `order_id` int() NOT NULL,
  `order_num` int() DEFAULT NULL,
  `order_type` int() 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(,,),(,,),(,,),(,,);

先执行事务(事务132868)：

BEGIN;
update tb1001
set order_num=
where order_type=;

再执行事务(事务ID 132869)：

BEGIN;
update tb1001
set order_type=
where order_id=;

查看事务锁和阻塞信息

SELECT * FROM INNODB_LOCK_WAITS \G
*************************** . row ***************************
requesting_trx_id:
requested_lock_id: :::
  blocking_trx_id:
 blocking_lock_id: :::

select * from INNODB_LOCKS \G
*************************** . row ***************************
    lock_id: :::
lock_trx_id:
  lock_mode: X,GAP
  lock_type: RECORD
 lock_table: `db002`.`tb1001`
 lock_index: idx_order_type
 lock_space:
  lock_page:
   lock_rec:
  lock_data: ,
*************************** . row ***************************
    lock_id: :::
lock_trx_id:
  lock_mode: X
  lock_type: RECORD
 lock_table: `db002`.`tb1001`
 lock_index: idx_order_type
 lock_space:
  lock_page:
   lock_rec:
  lock_data: , 

事务132868上锁信息：

---TRANSACTION , ACTIVE  sec
 lock struct(s), heap size ,  row lock(s), undo log entries
MySQL thread id , OS thread handle , query id  127.0.0.1 admin
TABLE LOCK table `db002`.`tb1001` trx id  lock mode IX

RECORD LOCKS space id  page no  n bits  index idx_order_type of table `db002`.`tb1001` trx id  lock_mode X
Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex 73757072656d756d; asc supremum;;

Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

RECORD LOCKS space id  page no  n bits  index PRIMARY of table `db002`.`tb1001` trx id  lock_mode X locks rec but not gap
Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc       ;;
 : len ; hex 24000000230a28; asc $   # (;;
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc       ;;
 : len ; hex 24000000230a49; asc $   # I;;
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

事务132869上锁信息：

---TRANSACTION , ACTIVE  sec updating or deleting
mysql tables in use , locked
LOCK WAIT  lock struct(s), heap size ,  row lock(s), undo log entries
MySQL thread id , OS thread handle , query id  127.0.0.1 admin updating
update tb1001 set order_type= where order_id=
------- TRX HAS BEEN WAITING  SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id  page no  n bits  index idx_order_type of table `db002`.`tb1001` trx id  lock_mode X locks gap before rec insert intention waiting
Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

------------------
TABLE LOCK table `db002`.`tb1001` trx id  lock mode IX
RECORD LOCKS space id  page no  n bits  index PRIMARY of table `db002`.`tb1001` trx id  lock_mode X locks rec but not gap
Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc       ;;
 : len ; hex 250000002407c6; asc %   $  ;;
 : len ; hex 8000000a; asc     ;;
 : len ; hex ; asc     ;;

RECORD LOCKS space id  page no  n bits  index idx_order_type of table `db002`.`tb1001` trx id  lock_mode X locks gap before rec insert intention waiting
Record lock, heap no  PHYSICAL RECORD: n_fields ; compact format; info bits
 : len ; hex ; asc     ;;
 : len ; hex ; asc     ;;

由于执行事务(事务132868)按照order_type=2条件更新，因此先使用索引idx_order_type定位到order_type=2的记录并加锁(ROW LOCK)，再根据二级索引上包含的主键索引值找到表上order_id=2和order_id=4的记录并加锁(ROW LOCK)，加锁如下：

由于执行事务(事务132869)按照order_id=3条件更新，先根据主键定位到order_id=3并加锁(ROW LOCK)，然后根据主键中数据(order_type=1+ order_id=3)到索引idx_order_type上找到满足条件的记录并加锁，UPDATE操作将数据(1,3)更新为(2,3)，因此会将索引idx_order_type上记录(1,3)标记为删除，然后在记录(2,2)和(2,4)之间插入新记录(2,3),在插入记录前，为防止其他事务在该物理位置上插入其他数据，需要先在索引idx_order_type上申请记录(2,2)和(2,4)之间插入意向锁(Insert Intention Gap Lock),其加锁如下：

而由于索引记录(2,4)上已被事务132868加锁（X LOCK+ ROW LOCK）,因此导致加插入意向锁(Insert Intention Gap Lock)被阻塞，处于“lock_mode X locks gap before rec insert intention waiting”的等待状态。