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  • MySQL 间隙锁

    一.根据案例二：不同索引加锁顺序的问题，模拟重现死锁（详细操作步骤）

    • 1.RR级别下，更新操作默认会加行级锁，行级锁会对索引加锁
    • 2.如果更新语句使用多个索引，行级锁会先锁定普通索引，再锁定聚簇索引
    • 3.如果两个SQL用到了不同的普通索引，或者一个用了，另外一个没用
    • 4.会导致这两个SQL加行级锁的顺序不一致，形成多个事物之间X锁的循环等待，形成死锁
    1.1表结构
    root@slave01 22:28:  [sqltest]> create table user_info (id int primary key,username varchar(20), status tinyint,key(username));
Query OK, 0 rows affected (0.03 sec)
#
root@slave01 22:54:  [sqltest]> show create table user_info;
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                                                                                            |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user_info | CREATE TABLE `user_info` (
  `id` int(11) NOT NULL,
  `username` varchar(20) DEFAULT NULL,
  `status` tinyint(4) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
#
insert into user_info values(1,'yzw1',1);
insert into user_info values(3,'yzw3',0);
insert into user_info values(5,'yzw5',1);
    1.2执行计划
    root@master 23:15:  [sqltest]> desc update user_info set status=1 where username='yzw3';
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | UPDATE      | user_info | NULL       | range | username      | username | 63      | const |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set (0.01 sec)
#
root@master 23:15:  [sqltest]> desc update user_info set username='yzw33' where id=3;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | UPDATE      | user_info | NULL       | range | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set (0.00 sec)
    1.3重现
    session 1 session 2
    begin;
    update user_info set status=1 where username='yzw3';    		  
    此时session除了给索引username加x锁，还给主键索引id=3的行记录加了x锁，但是这两个锁并不是同时获取的  
    第一步先获取普通索引username上的X锁  
    此时session 2进行更新 begin;
    update user_info set username='yzw33' where id=3;
      session 2
    第一步需要先获取聚簇索引，也就是主键索引上的X锁
    第二步需要获取普通索引username上的X锁
      出现死锁ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    需要session 2的主键索引X锁 需要session 1的普通索引username X锁 
    root@master 23:11:  [(none)]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table            | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| 48736590:1135:3:3 | 48736590    | X         | RECORD    | `sqltest`.`user_info` | PRIMARY    |       1135 |         3 |        3 | 3         |
| 48733648:1135:3:3 | 48733648    | X         | RECORD    | `sqltest`.`user_info` | PRIMARY    |       1135 |         3 |        3 | 3         |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)
    1.4重现
    • 1.给status列增加索引
    create index idx_status on user_info(status);
    • 2.执行计划，两条语句都是走的普通索引
    root@master 23:26:  [sqltest]> desc update user_info set status=1 where username='yzw3';
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | UPDATE      | user_info | NULL       | range | username      | username | 63      | const |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set (0.04 sec)
#
root@master 23:29:  [sqltest]> desc update user_info set username='yzw33' where status=0;
+----+-------------+-----------+------------+-------+---------------+------------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key        | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+------------+---------+-------+------+----------+-------------+
|  1 | UPDATE      | user_info | NULL       | range | idx_status    | idx_status | 2       | const |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+------------+---------+-------+------+----------+-------------+
1 row in set (0.01 sec)

    • 3.两个会话更新索引

      session 1 session 2
      begin;
      update user_info set status=1 where username='yzw3';      		  
      第一步已经获取了username的普通索引X锁 begin;
      update user_info set username='yzw33' where status=0;
        第一步已经获取status的普通索引X锁
      第二步给主键索引加X锁
      第二步需要给主键索引加X锁，无法获取 出现死锁ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
        因为要更新非主键索引username，因此要给username加X锁，但是无法获取 
      root@master 23:30:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table            | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| 48739611:1135:5:2 | 48739611    | X         | RECORD    | `sqltest`.`user_info` | idx_status |       1135 |         5 |        2 | 0, 3      |
| 48738737:1135:5:2 | 48738737    | X         | RECORD    | `sqltest`.`user_info` | idx_status |       1135 |         5 |        2 | 0, 3      |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)
    1.5多查询一次取主键，解决死锁
    begin; 
update user_info set status=1 where id in (select id  from (select id from user_info where username='yzw3') t);
begin;
update user_info set username='yzw33' where id in (select id from  (select id from user_info where status=0) t);

    root@master 23:56:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table            | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
| 48744368:1135:3:3 | 48744368    | X         | RECORD    | `sqltest`.`user_info` | PRIMARY    |       1135 |         3 |        3 | 3         |
| 48744353:1135:3:3 | 48744353    | X         | RECORD    | `sqltest`.`user_info` | PRIMARY    |       1135 |         3 |        3 | 3         |
+-------------------+-------------+-----------+-----------+-----------------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)

    > 依然存在死锁，因为都给主键id加了X锁

    二.根据方案三：TMS死锁分析和处理，模拟重现死锁（详细操作步骤）

    • 1.构造数据
    create table user_info1 (id int primary key,name varchar(10), level tinyint);
insert into user_info1 values(1,'AA',0);
insert into user_info1 values(3,'CC',0);
insert into user_info1 values(5,'EE',2);
insert into user_info1 values(7,'GG',5);
insert into user_info1 values(9,'GG',7);
insert into user_info1 values(11,'JJ',20);
session 1 >select * from user_info1;
+----+------+-------+
| id | name | level |
+----+------+-------+
|  1 | AA   |     0 |
|  3 | CC   |     0 |
|  5 | EE   |     2 |
|  7 | GG   |     5 |
|  9 | GG   |     7 |
| 11 | JJ   |    20 |
+----+------+-------+
6 rows in set (0.00 sec)

    • 2.过程

      session 1 session 2
        begin;
      insert into user_info1 values(2,'BB',1);
      delete from user_info1 where name='BB';
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction      		 此时是RC级别 session 2首先插入数据并给这一行加X锁，因为没有索引，同时在两边加gap锁
      delete from user_info1 where name='BC';
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction      		  
      delete from user_info1 where level=8;
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction      		  
      delete where条件没有索引，会对整表扫描并加gap锁，但是此时session2已经加了gap锁，session1的delete事务被回退   
      delete from user_info1 where  name='BB'
------- TRX HAS BEEN WAITING 10 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1137 page no 3 n bits 80 index PRIMARY of table `sqltest`.`user_info1` trx id 48906774 lock_mode X locks rec but not gap waiting
Record lock, heap no 8 PHYSICAL RECORD: n_fields 5; compact format; info bits 0

    • 3保持RCset tx_isolation='READ-COMMITTED';级别，增加索引

      session 1 >create index idx_name_level on user_info1(name,level);
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0
      session 1 session 2
        begin;
      insert into user_info1 values(2,'BB',1);
      delete from user_info1 where name='BB';
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction      		 此时是RC级别 session 2首先插入数据并加gap锁，
      session 1 >delete from user_info1 where name='BC';
      Query OK, 0 rows affected (0.00 sec)      		 RC级别下，增加了索引，没有了gap锁
      session 1 >delete from user_info1 where name='BB';
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
      session 1 >delete from user_info1 where level=8;
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
      session 1 >delete from user_info1 where level=20;
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
      session 1 >delete from user_info1 where level=7;
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
      session 1 >delete from user_info1 where level=2;
      ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
      		 索引(a,b)，如果where字段使用了索引a，则不存在gap锁了，猜测是单独使用索引b还是会存在gap锁，因为无法使用索引 
      root@master 16:31:  [(none)]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+------------------------+------------+------------+-----------+----------+-----------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table             | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------+-------------+-----------+-----------+------------------------+------------+------------+-----------+----------+-----------+
| 48942672:1137:3:8 | 48942672    | X         | RECORD    | `sqltest`.`user_info1` | PRIMARY    |       1137 |         3 |        8 | 2         |
| 48942589:1137:3:8 | 48942589    | S         | RECORD    | `sqltest`.`user_info1` | PRIMARY    |       1137 |         3 |        8 | 2         |
+-------------------+-------------+-----------+-----------+------------------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)

    • 4增加索引(b)，验证

    session 1 >create index idx_level on user_info1(level);
Query OK, 0 rows affected (0.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

    session 1 | session 2
---|---
&nbsp; | begin;</br>insert into user_info1 values(2,'BB',1);
delete from user_info1 where  name='BB';</br>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction | 此时是RC级别 session 2首先插入数据并加X锁
session 1 >delete from user_info1 where name='BC';</br>Query OK, 0 rows affected (0.00 sec)</br>session 1 >delete from user_info1 where name='BB';</br>ERROR 1205 (HY000): Lock wait timeout xceeded; try restarting transaction</br>session 1 >delete from user_info1 where level=8;</br>Query OK, 0 rows affected (0.01 sec)</br>session 1 >delete from user_info1 where level=20;</br>Query OK, 1 row affected (0.01 sec)</br>session 1 >delete from user_info1 where level=7;</br>Query OK, 1 row affected (0.00 sec)</br>session 1 >delete from user_info1 where level=2;</br>Query OK, 1 row affected (0.00 sec)</br>|此时b有了索引，RC级别下也就不存在gap锁了
    • 5更改为RR级别set tx_isolation='REPEATABLE-READ';，先使用索引(a,b)
    session 2 >drop index idx_level on user_info1;
Query OK, 0 rows affected (0.16 sec)
Records: 0  Duplicates: 0  Warnings: 0
session 2 >begin;
Query OK, 0 rows affected (0.00 sec)
#
session 2 >insert into user_info1 values(2,'BB',1);
Query OK, 1 row affected (0.00 sec)
#
session 1 >delete from user_info1 where name='BB';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
session 1 >delete from user_info1 where name='BC';
Query OK, 0 rows affected (0.00 sec)
#
session 1 >delete from user_info1 where level=8;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
session 1 >delete from user_info1 where level=2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
session 1 >delete from user_info1 where name='CB';
Query OK, 0 rows affected (0.00 sec)
#
session 1 >delete from user_info1 where name='BA';
Query OK, 0 rows affected (0.00 sec)
    • 6总结
    1. RC和RR级别下如果更新没有索引的字段，会加gap锁
    2. 必须能用上索引，(a,b)只用b是无效索引
    3. RC增加了索引后就不存在gap锁了
    4. RR级别在没有索引的情况，用不上索引也会产生gap锁
    5. 解决方法，最好使用RC级别+索引，或者RR下修改参数innodb_locks_unsafe_for_binlog=on
    6. 间隙锁只会block住insert操作
    session 1 >select * from user_info1;
+----+------+-------+
| id | name | level |
+----+------+-------+
|  1 | AA   |     0 |
|  3 | BA   |     1 |
|  5 | EE   |     2 |
|  7 | GG   |     5 |
|  9 | GG   |     7 |
| 11 | JJ   |    20 |
+----+------+-------+
6 rows in set (0.00 sec)
#
session 1 >begin;
Query OK, 0 rows affected (0.00 sec)
#
session 1 >update user_info1 set level=3 where name='BA';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0
session 2 >begin;
Query OK, 0 rows affected (0.01 sec)
#
session 2 >insert into user_info1 values(2,'AB',1);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

    三.根据案例四：MySQL间隙锁导致的Deadlock分析，模拟重现死锁（详细操作步骤）

    3.1 造数据
    create table t3 (id int auto_increment,coupon_id int,uid int,is_inuse tinyint,primary key(id),key (coupon_id,uid));
insert into t3 (coupon_id,uid,is_inuse) values(16,160825,1);
insert into t3 (coupon_id,uid,is_inuse) values(16,160835,1);
insert into t3 (coupon_id,uid,is_inuse) values(16,160845,1);
insert into t3 (coupon_id,uid,is_inuse) values(16,160855,1);
insert into t3 (coupon_id,uid,is_inuse) values(16,160865,1);
root@master 22:56:  [sqltest]> select * from t3;
+----+-----------+--------+----------+
| id | coupon_id | uid    | is_inuse |
+----+-----------+--------+----------+
|  1 |        16 | 160825 |        1 |
|  2 |        16 | 160835 |        1 |
|  3 |        16 | 160845 |        1 |
|  4 |        16 | 160855 |        1 |
|  5 |        16 | 160865 |        1 |
+----+-----------+--------+----------+
5 rows in set (0.00 sec)
    3.2 行记录两边间隙锁范围内无法插入
    • 1.session 1给id=3的行记录加X锁，同时加间隙锁：160835160845，160845160855
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
select * from t3 where (uid=160845 and coupon_id=16 and is_inuse=1) for update;
+----+-----------+--------+----------+
| id | coupon_id | uid    | is_inuse |
+----+-----------+--------+----------+
|  3 |        16 | 160845 |        1 |
+----+-----------+--------+----------+
1 row in set (0.00 sec)
    • 2.session 2插入记录，在间隙范围：160835~160845
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
insert into t3 (coupon_id,uid,is_inuse) values (16,160840,1);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    • 3.lock
    root@master 23:08:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data     |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| 49020303:1139:4:4 | 49020303    | X,GAP     | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        4 | 16, 160845, 3 |
| 49020011:1139:4:4 | 49020011    | X         | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        4 | 16, 160845, 3 |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
2 rows in set, 1 warning (0.00 sec)
    • 4.session 2插入记录，在间隙范围：160845~160855
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
insert into t3 (coupon_id,uid,is_inuse) values (16,160850,1);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    • 5.lock
    root@master 23:11:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data     |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| 49022356:1139:4:5 | 49022356    | X,GAP     | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        5 | 16, 160855, 4 |
| 49020011:1139:4:5 | 49020011    | X,GAP     | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        5 | 16, 160855, 4 |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
2 rows in set, 1 warning (0.00 sec)
    3.3 行记录间隙锁范围外可以插入
    ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
session 2 >insert into t3 (coupon_id,uid,is_inuse) values (16,160860,1);
Query OK, 1 row affected (0.01 sec)
    3.4 update不存在的记录，也会加间隙锁
    • 1.数据
    session 1 >select * from t3;
+----+-----------+--------+----------+
| id | coupon_id | uid    | is_inuse |
+----+-----------+--------+----------+
|  1 |        16 | 160825 |        1 |
|  2 |        16 | 160835 |        1 |
|  3 |        16 | 160845 |        1 |
|  4 |        16 | 160855 |        1 |
|  5 |        16 | 160865 |        1 |
+----+-----------+--------+----------+
5 rows in set (0.00 sec)
    • 2.update一条不存在的记录，此时gap锁范围：160835160845，160845160855
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
update t3 set is_inuse=0 where coupon_id=16 and uid=160845 and is_inuse=1;
    • 3.在间隙范围插入新值会产生死锁
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
insert into t3 (coupon_id,uid,is_inuse) values (16,160840,1);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    • 4.lock
    root@master 23:11:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data     |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
| 49023844:1139:4:4 | 49023844    | X,GAP     | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        4 | 16, 160845, 3 |
| 49023813:1139:4:4 | 49023813    | X         | RECORD    | `sqltest`.`t3` | coupon_id  |       1139 |         4 |        4 | 16, 160845, 3 |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+---------------+
2 rows in set, 1 warning (0.00 sec)
    3.5删除索引，即使是间隙范围外的插入也会被block
    • 1.删除索引
    session 2 >drop index coupon_id on t3;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0
#
session 2 >commit;
Query OK, 0 rows affected (0.00 sec)
    • 2.事务1加锁
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
select * from t3 where (uid=160845 and coupon_id=16 and is_inuse=1) for update;
    • 3.事务2在间隙锁范围外插入，此时不是间隙锁，而是全表锁，无法插入
    set tx_isolation='REPEATABLE-READ';
set autocommit=0;
start transaction;
select * from t3 where (uid=160850 and coupon_id=16 and is_inuse=1) for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    • 4.lock
    root@master 23:27:  [sqltest]> select * from information_schema.innodb_locks;
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| lock_id           | lock_trx_id | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| 49025074:1139:3:2 | 49025074    | X         | RECORD    | `sqltest`.`t3` | PRIMARY    |       1139 |         3 |        2 | 1         |
| 49025016:1139:3:2 | 49025016    | X         | RECORD    | `sqltest`.`t3` | PRIMARY    |       1139 |         3 |        2 | 1         |
+-------------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)
    3.6 总结
    • 1.RR级别下，有索引，更新的时候会给行加X锁，前后加GAP锁
    • 2.gap锁范围内无法插入新数据，避免产生幻读
    • 3.gap锁范围外可以正常插入新数据
    • 4.如果没有索引，更新的事务则加的就是全表锁了
    • 5.间隙内或者间隙外都无法插入新数据
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  • 原文地址：https://www.cnblogs.com/jenvid/p/9074780.html
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