Mysql database lock learning notes

What is a lock

A lock is a mechanism by which a computer coordinates multiple processes or threads to access a resource concurrently , Used to manage concurrent access to shared resources

Table lock

Table lock , Lock the whole table with each operation . Large locking size , The highest probability of lock collisions , Lowest degree of concurrency . Apply to MyISAM、 InnoDB Wait for the storage engine

Table locks

• Watch reading lock ( grammar ：lock tables Table name read; )
• Watch write lock ( grammar ：lock tables Table name write; )
• Release the lock （unlock tables）

Read the lock ：


Write lock ：

Metadata lock

meta data lock , Metadata lock , Abbreviation MDL.

MDL The main function of locks is to maintain Data consistency of table metadata , When there are transactions on the table , The table structure cannot be modified . for fear of DML And DDL Conflict , Ensure the correctness of reading and writing

Intent locks

for fear of DML When executed , Conflict between row lock and table lock , stay InnoDB Intention lock is introduced in , So that the table lock does not need to check whether each row of data is locked , Use intent lock to reduce the checking of table lock .

• Intention sharing lock (IS)： By statement select … lock in share mode add to ; And Table locks share locks (read) compatible , Exclusive lock with watch lock (write) Mutually exclusive .
• Intending to be exclusive (IX)： from insert、update、delete、select…for update add to ; Share lock with table lock (read) And exclusive lock (write) Are mutually exclusive , Intent locks are not mutually exclusive .

Once the transaction is committed , Intention sharing lock 、 The intentional exclusive lock will be automatically cancelled

Row-level locks

Row-level locks , Each operation locks the corresponding row data . Locking granularity minimum , The lowest probability of lock collisions , Highest concurrency . Apply to InnoDB In the storage engine .

InnoDB The data is organized based on indexes , Row locking is achieved by locking the index items on the index , Instead of adding to the record lock . For row level locks , It is mainly divided into the following three categories ：

Row lock

• Shared lock （S）： Allow a transaction to read a line , Prevent other transactions from obtaining exclusive locks on the same dataset .

• Exclusive lock （X）： Allow to get transaction update data of exclusive lock , Prevent other transactions from obtaining shared and exclusive locks of the same dataset .

• When searching for a unique index , When performing equivalence matching on existing records , It will be automatically optimized as row lock .

• InnoDB The row lock of is a lock added to the index , Retrieve data without index criteria , that InnoDB All items in the table will be marked Record and lock , here Will be upgraded to table lock .

Clearance lock & Temporary key lock

• Equivalent query on Index ( unique index ), When locking records that do not exist , Optimized for clearance lock .
• Equivalent query on Index ( It's not the only common index ), When traversing to the right, the last value does not meet the query requirements ,next-key lock Degenerate to clearance lock .
• Range query on Index ( unique index )– Access to the first value that does not satisfy the condition .

Be careful ： The only purpose of a gap lock is to prevent other transactions from inserting gaps . Gap locks can coexist , The gap lock adopted by a transaction will not Prevent another transaction from adopting gap lock on the same gap .

The only purpose of a gap lock is to prevent other transactions from inserting gaps . Gap locks can coexist , A gap lock adopted by one transaction will not prevent another transaction from adopting a gap lock on the same gap .