MySQL row-level locks (row locks, adjacent key locks, gap locks)

Third, row-level lock

Row-level lock, each operation locks the corresponding row data.The lock granularity is the smallest, the probability of lock conflict is the lowest, and the concurrency is the highest.Applied in the InnoDB storage engine.
InnoDB's data is organized based on indexes, and row locks are implemented by locking the index items on the index instead of locking records.

Row-level locks are mainly divided into the following three categories:
1. Record Lock: locks a single row record to prevent other transactions from updating anddelete.It is supported under RC and RRisolation level.
2. Gap Lock: Lock the index record gap (excluding this record) to ensure that the index record gap remains unchanged, preventing other transactions from inserting in this gap, resulting in phantom reading.Both are supported under the RR isolation level.
3. Next-Key Lock: a combination of row lock and gap lock, locks data at the same time, and locks the gap in front of the data.Supported under RR isolation level.

1, row lock

InnoDB implements the following two types of row locks:

1. Shared lock(S): Allows a transaction to read a row, preventing other transactions from acquiring an exclusive lock on the same data set.
2. Exclusive lock (X): Allows transactions that acquire exclusive locks to update data, preventing other transactions from acquiring shared locks and exclusive locks for the same data set.

Demo

By default, InnoDB runs at the REPEATABLE READ transaction isolation level, and InnoDB uses next-key locks for searches and index scans to prevent phantom reads.

1. When retrieving against a unique index, it will automatically optimize to row locks when performing equal value matching on existing records.

case1: select normal
case2: select…lock in sharemode

case3: shared and exclusive locks
case4: Exclusive and exclusive locks

2. InnoDB row locks are for index locks and do not retrieve data through index conditions.Then InnoDB will lock all records in the table, and it will be upgraded to table lock at this time.

View intent locks and row locks: select object_schema,object_name,index_name,lock_type,lock_mode,lock_data from performance_schema.data_locks;

2, gap lock/pro-key lock

By default, InnoDB runs at the REPEATABLE READ transaction isolation level, and InnoDB uses next-key locks for searches and index scans to prevent phantom reads.

1. The equivalent query (unique index) on the index, when locking the non-existing record, is optimized as a gap lock.
2. Equal value query on the index (ordinary index), when the last value does not meet the query requirements when traversing to the right, the next-key lock degenerates into a gap lock.
3. Range queries on indexes (unique indexes) – will access until the first value that does not satisfy the condition.

Note: The only purpose of a gap lock is to prevent other transactions from inserting into the gap.Gap locks can coexist, and a gap lock taken by one transaction does not prevent another transaction from taking a gap lock on the same gap.

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