Gap locks

Gap Locks（ Clearance lock ） Used to lock records within a certain range . It blocks intervals in index records , Or the range before the first index record , Or the range after the last index record . It is mainly used for Prevent other transactions from inserting data inside the interval in the index record , And cause unreal reading . about Read submitted (Read Committed, RC) Transaction isolation level ,Gap Locks Will automatically fail .

Data locking range analysis

         hypothesis user In the table user_id by Primary key (Primary Key) And for unsigned int  type .

select * from user where user_id < 7 for update;

The record range locked by the above operation is user_id >= 0 To user_id = 6 The record of , Only after the transaction is committed , Only other transactions can operate records in this range . Notice that even user_id=3 The corresponding record does not exist , Before the transaction is committed , Insert user_id=3 Records are also not allowed , Otherwise, there is no way to solve the problem of unreal reading .

select * from user where user_id > 7 and user_id < 1000 for update;

The record range locked by the above operation is user_id > 7 To user_id < 10000 The record of , Only after the transaction is committed , Only other transactions can operate records in this range .

select * from user where user_id > 1000 for update;

The record range locked by the above operation is user_id > 1000 To int Record between maximum values , Only after the transaction is committed , Only other transactions can operate records in this range .