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In MySQL5.7, when READ COMMITTED isolation level is used, or the deprecated innodb_locks_unsafe_for_binlogsystem variable is enabled, there is no InnoDB gap locking exceptfor foreign-key constraint checking and duplicate-key checking.Also, record locks for nonmatching rows arereleased after MySQL has evaluatedthe WHERE condition.
经验证手册里的东西有误。其实，测试情况并不是这样，重复键检查和外键检查并不是一定加gap锁。
外键检查
子表中更新的值父表没有，对父表加间隙锁
1、 MariaDB [mytest]> update t4 set id=3 where id=1;//insert同
2、 MariaDB [mytest]> show engine innodb status\G
------------
TRANSACTIONS
------------
……
4 lockstruct(s), heap size 320, 2 row lock(s)
MySQLthread id 2, OS thread handle 0xa63a6b90, query id 118 localhost root init
showengine innodb status
Trx readview will not see trx with id >= 270882, sees < 270882
TABLELOCK table `mytest`.`t4` trx id 270881 lock mode IX
RECORDLOCKS space id 75 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t4`trx id 270881 lock_mode X locks rec but not gap
Recordlock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 00000004221c; asc " ;;
2: len 7; hex 9a000001530110; asc S ;;
3: len 4; hex 80000001; asc ;;
TABLE LOCKtable `mytest`.`t3` trx id 270881 lock mode IS
RECORDLOCKS space id 74 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t3`trx id 270881 lock mode S locks gap before rec
Recordlock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000004; asc ;;
1: len 6; hex 000000042216; asc " ;;
2: len 7; hex 960000014f0128; asc O (;;
3: len 4; hex 80000004; asc ;;
----------------------------
END OF INNODB MONITOR OUTPUT
子表更新，父表有值，父表加记录锁
1、 MariaDB [mytest]> update t4 set id=4 where id=1;//insert同
2、 MariaDB [mytest]> show engine innodb status\G
…
------------
TRANSACTIONS
------------
…
4 lock struct(s),heap size 320, 2 row lock(s), undo log entries 2
MySQLthread id 2, OS thread handle 0xa63a6b90, query id 145 localhost root init
showengine innodb status
TABLELOCK table `mytest`.`t4` trx id 270887 lock mode IX
RECORDLOCKS space id 75 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t4`trx id 270887 lock_mode X locks rec but not gap
Recordlock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000042227; asc "‘;;
2: len 7; hex 1f000001fb0bab; asc ;;
3: len 4; hex 80000001; asc ;;
TABLELOCK table `mytest`.`t3` trx id 270887 lock mode IS
RECORDLOCKS space id 74 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t3`trx id 270887 lock mode S locks rec but not gap
Recordlock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000004; asc ;;
1: len 6; hex 000000042216; asc " ;;
2: len 7; hex 960000014f0128; asc O (;;
3: len 4; hex 80000004; asc ;;
----------------------------
END OFINNODB MONITOR OUTPUT
============================
主键检查：不会加间隙锁
MariaDB[mytest]> create table t5 (id int primary key,id1 int);
Query OK,0 rows affected (0.11 sec)
MariaDB[mytest]> insert into t5 values(1,1),(3,3),(5,5);
Query OK,3 rows affected (0.01 sec)
Records:3 Duplicates: 0 Warnings: 0
Insert、update
有值，则不是间隙。
MariaDB[mytest]> update t5 set id=1 where id=5;
------------
TRANSACTIONS
------------
3 lockstruct(s), heap size 320, 2 row lock(s)
MySQLthread id 2, OS thread handle 0xa63a6b90, query id 168 localhost root init
showengine innodb status
Trx readview will not see trx with id >= 270907, sees < 270907
TABLELOCK table `mytest`.`t5` trx id 270906 lock mode IX
RECORDLOCKS space id 76 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t5`trx id 270906 lock_mode X locks rec but not gap
Recordlock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000005; asc ;;
1: len 6; hex 000000042234; asc "4;;
2: len 7; hex a9000001620128; asc b (;;
3: len 4; hex 80000005; asc ;;
RECORDLOCKS space id 76 page no 3 n bits 72 index `PRIMARY` of table `mytest`.`t5`trx id 270906 lock mode S locks rec but not gap
Recordlock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000042234; asc "4;;
2: len 7; hex a9000001620110; asc b ;;
3: len 4; hex 80000001; asc ;;
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原文地址：http://blog.csdn.net/yanzongshuai/article/details/45442245
