Mysql lock mechanism -- read lock

===============

1 Prepare data

1.1 Create a table

1.1.1 Create the Employee table

DROP TABLE IF EXISTS employee;
CREATE TABLE IF NOT EXISTS employee (
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(40),
    dept_id INT
)engine myisam;

1.1.2 Create the Department table

DROP TABLE IF EXISTS department;
CREATE TABLE IF NOT EXISTS department (
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(40)
)engine myisam;

1.1.3 Note: The storage engines of the above two tables are MyISam

1.2 Insert data

INSERT INTO employee(name, dept_id) VALUES('Alice', 1);
INSERT INTO employee(name, dept_id) VALUES('Bob', 1);

INSERT INTO department(name) VALUES('RD');

2 test

2.1 Add a read lock to the Employee table

LOCK TABLE employee READ;

2.2 Query the Employee table

2.2.1 Current session (terminal, command line window)

Note: The current session refers to the session (window, command line) that executed the LOCK TABLE employee READ statement above; in my case, the white background .

SELECT * FROM employee;

2.2.2 Other sessions (terminal, command line window)

Note: Open a new session window; in my case, a black background .

2.2.3 Conclusion

It can be seen that after adding a read lock, both the current session and other sessions can perform read operations, that is, read locks are shared for read operations .

2.3 Update the Employee table

2.3.1 Current session

UPDATE employee SET name = 'Alice02' WHERE id = 1;

2.3.2 Other sessions

Result: suspended, blocked (above)

Next, execute the UNLOCK TABLES command in the 'current session' to unlock the lock (above)

After unlocking, the update statement in 'other session' is executed immediately; as you can see, it takes 1 min 5.11 sec (above)

2.3.3 Conclusion

For update operations:

The current session cannot be updated (understanding: adding a read lock, of course, cannot perform write (update) operations)
Other sessions must wait until the current session is unlocked before updating, otherwise it will hang (after all, the 'current session' has a lock, and 'other sessions' must not be able to write)

2.4 Reading other tables

2.4.0 Preparation

Hint: The 'current session' needs to lock the Employee table again (because UNLOCK TABLES was executed above)

LOCK TABLE employee READ;

2.4.1 Current session

SELECT * FROM department;

2.4.2 Other sessions

2.4.3 Conclusion

For read operations from other tables:

The current session cannot be read (the Department table is not locked and cannot be read)
Other sessions can read normally

3 Conclusion

Read lock, for the current session (Owner), the locked table can be read, but it cannot be updated, nor can it read other tables;

For other sessions (Other), the locked table can be read, the update will be blocked, and other tables can be read normally;

Read locks are shared for read operations;

Mysql lock mechanism -- read lock

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