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MySQL InnoDB中唯一索引和非唯一索引時的加鎖情況
MySQL的默認隔離級別為RR，這篇文章討論一下唯一索引和非唯一索引時，使用lock read（consistent read不加鎖，不討論）時的不同的加鎖策略。 前提條件是使用了 RR隔離級別,並且使用了 索引掃描。
假設有以下表的定義和數據：
mysql> create table test1 (year int);
mysql> insert into test1 values(2010),(2007),(2005),(2012),(2000),(2017);
mysql> select * from test1;
+------+
| year |
+------+
| 2010 |
| 2007 |
| 2005 |
| 2012 |
| 2000 |
| 2017 |
+------+
6 rows in set (0.00 sec)
Session 1:
mysql> start transaction;
mysql> select * from test1 where year between 2007 and 2010 for update;
+------+
| year |
+------+
| 2007 |
| 2010 |
+------+
2 rows in set (0.00 sec)
Session 2:
mysql> insert into test1 values(2004); =>Query OK,
mysql> delete from test1 where year=2005; =>Query OK,
mysql> insert into test1 values(2005); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2006); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2008); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; => ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2012); =>Query OK,
加鎖范圍： (2005,2012], 也就是 在2007,2010,2012上各加了一個Next key lock.
========
show engine innodb status\G 顯示如下，可以看到有索引idx上有3個x鎖（由於idx非clustered index，所以clustered index上也要加3行鎖）
MySQL thread id 11, OS thread handle 139829643007744, query id 257 localhost root
TABLE LOCK table `sample`.`test1` trx id 156570 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156570 lock_mode X
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007d7; asc ;;
1: len 6; hex 0000009a8a12; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007dc; asc ;;
1: len 6; hex 0000009a8a14; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156570 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a12; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de00000166011f; asc f ;;
3: len 4; hex 800007d7; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a14; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de00000166013d; asc f =;;
3: len 4; hex 800007dc; asc ;;
=========
mysql> start transaction;
mysql> select * from test1 where year =2010 for update;
+------+
| year |
+------+
| 2010 |
+------+
1 row in set (0.00 sec)
====
Session 2:
mysql> delete from test1 where year=2007 => Query OK
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2009); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; => Query OK
加鎖范圍： (2007,2012),也就是在 2010上加了一個Next key lock，在2011后上加了一個gap鎖：
4 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 11, OS thread handle 139829643007744, query id 262 localhost root
TABLE LOCK table `sample`.`test1` trx id 156571 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156571 lock_mode X
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156571 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156571 lock_mode X locks gap before rec
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007dc; asc ;;
1: len 6; hex 0000009a8a14; asc ;;
mysql> create unique index idx on test1(year);
Session 1:
mysql> start transaction;
mysql> select * from test1 where year between 2007 and 2010 for update;
+------+
| year |
+------+
| 2007 |
| 2010 |
+------+
2 rows in set (0.00 sec)
Session 2:
mysql> delete from test1 where year=2005; => Query OK
mysql> insert into test1 values(2006); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2008); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2012); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2013); =>Query OK
加鎖范圍： (2005,2012]和情形1一樣，不再重復
mysql> start transaction;
mysql> select * from test1 where year=2010 for update;
+------+
| year |
+------+
| 2010 |
+------+
1 row in set (0.00 sec)
Session 2:
mysql> insert into test1 values(2009); =>Query OK
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>Query OK
加鎖范圍： 僅僅是2010。
TABLE LOCK table `sample`.`test1` trx id 156595 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156595 lock_mode X locks rec but not gap
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156595 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
最后這一點和DB2是不同的，只有一條記錄的情況下，若是唯一索引，DB2里也只要在2010上加鎖，但2007~2010之間的數是不允許插入的。
總結一下就是，只有一種特殊情況：就是唯一索引且只有一條記錄的情況下，不加Gap鎖，其他情況都要加Gap鎖。（MySQL官方文檔的原文是 Gap locking is not needed for statements that lock rows using a unique index to search for a unique row.）
注1：在show engine innodb status\G輸出中:
如果是單純的record lock，顯示的是：locks rec but not gap
如果是單純的gap lock， 顯示的是：locks gap before rec
如果是gap+record，也就是next key lock，顯示的是：lock_mode X <空>
注2：RR隔離級別才有gap lock，Read Commited下沒有
MySQL的默認隔離級別為RR，這篇文章討論一下唯一索引和非唯一索引時，使用lock read（consistent read不加鎖，不討論）時的不同的加鎖策略。 前提條件是使用了 RR隔離級別,並且使用了 索引掃描。
假設有以下表的定義和數據：
mysql> create table test1 (year int);
mysql> insert into test1 values(2010),(2007),(2005),(2012),(2000),(2017);
mysql> select * from test1;
+------+
| year |
+------+
| 2010 |
| 2007 |
| 2005 |
| 2012 |
| 2000 |
| 2017 |
+------+
6 rows in set (0.00 sec)
1. 非唯一索引
mysql> create index idx on test1(year);Session 1:
mysql> start transaction;
mysql> select * from test1 where year between 2007 and 2010 for update;
+------+
| year |
+------+
| 2007 |
| 2010 |
+------+
2 rows in set (0.00 sec)
Session 2:
mysql> insert into test1 values(2004); =>Query OK,
mysql> delete from test1 where year=2005; =>Query OK,
mysql> insert into test1 values(2005); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2006); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2008); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; => ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2012); =>Query OK,
加鎖范圍： (2005,2012], 也就是 在2007,2010,2012上各加了一個Next key lock.
========
show engine innodb status\G 顯示如下，可以看到有索引idx上有3個x鎖（由於idx非clustered index，所以clustered index上也要加3行鎖）
MySQL thread id 11, OS thread handle 139829643007744, query id 257 localhost root
TABLE LOCK table `sample`.`test1` trx id 156570 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156570 lock_mode X
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007d7; asc ;;
1: len 6; hex 0000009a8a12; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007dc; asc ;;
1: len 6; hex 0000009a8a14; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156570 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a12; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de00000166011f; asc f ;;
3: len 4; hex 800007d7; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a14; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de00000166013d; asc f =;;
3: len 4; hex 800007dc; asc ;;
=========
圖示如下：
2. 非唯一索引，Unique search
Session 1:mysql> start transaction;
mysql> select * from test1 where year =2010 for update;
+------+
| year |
+------+
| 2010 |
+------+
1 row in set (0.00 sec)
====
Session 2:
mysql> delete from test1 where year=2007 => Query OK
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2009); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; => Query OK
加鎖范圍： (2007,2012),也就是在 2010上加了一個Next key lock，在2011后上加了一個gap鎖：
4 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 11, OS thread handle 139829643007744, query id 262 localhost root
TABLE LOCK table `sample`.`test1` trx id 156571 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156571 lock_mode X
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156571 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156571 lock_mode X locks gap before rec
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007dc; asc ;;
1: len 6; hex 0000009a8a14; asc ;;
========示意如下：
3. 唯一索引
mysql> drop index idx on test1;mysql> create unique index idx on test1(year);
Session 1:
mysql> start transaction;
mysql> select * from test1 where year between 2007 and 2010 for update;
+------+
| year |
+------+
| 2007 |
| 2010 |
+------+
2 rows in set (0.00 sec)
Session 2:
mysql> delete from test1 where year=2005; => Query OK
mysql> insert into test1 values(2006); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2007); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2008); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2012); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> delete from test1 where year=2012; =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2013); =>Query OK
加鎖范圍： (2005,2012]和情形1一樣，不再重復
4. 唯一索引，Unique search
Session 1:mysql> start transaction;
mysql> select * from test1 where year=2010 for update;
+------+
| year |
+------+
| 2010 |
+------+
1 row in set (0.00 sec)
Session 2:
mysql> insert into test1 values(2009); =>Query OK
mysql> insert into test1 values(2010); =>ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into test1 values(2011); =>Query OK
加鎖范圍： 僅僅是2010。
TABLE LOCK table `sample`.`test1` trx id 156595 lock mode IX
RECORD LOCKS space id 7429 page no 4 n bits 80 index idx of table `sample`.`test1` trx id 156595 lock_mode X locks rec but not gap
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 800007da; asc ;;
1: len 6; hex 0000009a8a11; asc ;;
RECORD LOCKS space id 7429 page no 3 n bits 80 index GEN_CLUST_INDEX of table `sample`.`test1` trx id 156595 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000009a8a11; asc ;;
1: len 6; hex 000000026385; asc c ;;
2: len 7; hex de000001660110; asc f ;;
3: len 4; hex 800007da; asc ;;
最后這一點和DB2是不同的，只有一條記錄的情況下，若是唯一索引，DB2里也只要在2010上加鎖，但2007~2010之間的數是不允許插入的。
總結一下就是，只有一種特殊情況：就是唯一索引且只有一條記錄的情況下，不加Gap鎖，其他情況都要加Gap鎖。（MySQL官方文檔的原文是 Gap locking is not needed for statements that lock rows using a unique index to search for a unique row.）
注1：在show engine innodb status\G輸出中:
如果是單純的record lock，顯示的是：locks rec but not gap
如果是單純的gap lock， 顯示的是：locks gap before rec
如果是gap+record，也就是next key lock，顯示的是：lock_mode X <空>
注2：RR隔離級別才有gap lock，Read Commited下沒有