Shared lock

Shared lock （Shared Lock）, also called S lock 、 Read the lock . For row lock .
When a transaction adds a read lock to the data , Other transactions can only add read locks to the locked data , You can't lock it （ Exclusive lock ）, So other transactions can only read , Can't write .

Mainly to support the scenario of concurrent reading , Write operation is not allowed while reading .

Lock mode ：
select * from T where id=1 lock in share mode;
Release mode ：
commit、rollback;