MySQL lock leak detection and defect filling

Preface

I saw it today This article —— Learn about common lock types I saw many locks I didn't know or have never seen , It is hereby recorded that .

Intent locks

Use scenarios / Birth background

stay innodb Under the engine , We have put row locks on a row of a table , At this time, you need to lock the table . At this time, the engine will traverse each row in the table one by one , Judge whether there is row lock , If yes, locking fails .
This is obviously inefficient , The intention lock was born .

Intent lock features

• Intentional locks are divided into read intentional locks (IS lock ) And freehand lock (IX lock ) And all are table level locks
• When a transaction needs to be row locked , Will first add an intention lock to the entire table , This solves the problem of traversing the entire table when adding a table lock , Greatly improved performance .
• Press the intention lock directly , Neither intent lock nor row lock is mutually exclusive .

Self increasing lock

Self increasing lock (AI lock ) Simply put, it is a lock used on auto incrementing columns , Mainly for insert statements , When a statement wants to insert data into a table , A self incrementing lock will be added first , Block other insert statements

Self incrementing locking features

• The self incrementing lock is incompatible with each other , The same table has only one auto increment lock
• The self incrementing lock does not comply with the two-phase protocol , When the insert statement succeeds , This self incrementing lock is automatically released
• Even if the transaction is rolled back , The value of the auto increment lock will not decrease back

Several strategies of self incrementing lock

The traditional auto increment strategy has a great impact on the concurrency of insert statements , therefore MySQL It's optimized , A lightweight mechanism is introduced mutex To replace the self incrementing lock .
Through parameters innodb_automic_lock_mode Appoint

• innodb_automic_lock_mode = 0 Indicates the use of a traditional self incrementing lock .

• innodb_automic_lock_mode=1 Means a compromise , According to the specific situation , It is highly possible to use traditional self incrementing locks , It can also be used mutex Lightweight self incrementing lock . The details are as follows
  1.simple insert When the execution engine can clearly know the number to insert by analyzing the statement insert The statement is simple insert. I will use mutex
  2. bulk insert It is impossible to know how many rows are inserted by analyzing the statement ( example ：INSERT INTO t VALUES SELECT * FROM t1) The insert statement for is bulk insert. In this case, traditional insert
  3. mixed insert You are not sure whether you need to allocate columns with sub value added ( example INSERT INTO table VALUES(1,‘a’), (NULL,‘b’), (5, ‘C’), (NULL, ‘d’) ), In this case, the insertion amount in the worst case will be obtained for allocation , Use mutex.

• innodb_automic_lock_mode=2 All use mutex

Row lock correlation

Record locks

It is the row lock we usually understand

Record lock characteristics

• When we lock the secondary index , It will be Lock the corresponding cluster index
• When the index is not available , A full scan will be performed , Lock all scanned rows . Due to the two-stage agreement , These locks are only released when the transaction is committed , Obviously, it will greatly affect the concurrency . therefore MySQL This mechanism is optimized , When the scanned line is server After the layer is judged as unqualified , It will immediately release the lock of changing profession .( This clearly undermines the two-phase agreement )

Clearance lock （gap lock）

Is a way to load a lock between two indexes , There are... In total when adding the current bid 15 rows , The statement to be operated covers all the records , Then its clearance lock can be added to (-∞,0）（0,5）（5,10）（10,15）（15,superNum） superNum yes innodb The maximum value automatically added to each table .

Next-Key lock

The combination of clearance lock and record lock is called next-key, It is the same as before , At this point, the record of the lock becomes (-∞,0]（0,5]（5,10]（10,15]（15,superNum] . Each lock section is open in front and closed in back , Avoid repeated locking and missing locking .

Clearance lock .Next-key lock characteristic

• There is no conflict between clearance lock and clearance lock
• Gap locking only conflicts with inserting statements into this interval .

Insert intention lock

Insert intent lock is a special kind of gap lock , Only in INSERT Statement

Insert Intention Lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap.
Multiple transactions , At the same index , When inserting records in the same range interval , If the insertion position does not conflict , Not blocking each other .