A Deep Dive into DML, DDL, and Internal Lock Mechanisms
DML Locks in InnoDB:
InnoDB provides locking on the row level, supporting different types of locks for different operations. These are often involved in Data Manipulation Language (DML) operations like INSERT, UPDATE, and DELETE.
- Shared locks (S): Many transactions can hold a shared lock for the same row simultaneously. A transaction that holds a shared lock on a row can only read the row.
- Exclusive locks (X): Only one transaction can hold an exclusive lock for a row. A transaction that holds an exclusive lock on a row can modify the row.
- Intention shared locks (IS): A transaction intending to set a shared lock on individual rows sets an intention shared lock on the table.
- Intention exclusive locks (IX): A transaction intending to set an exclusive lock on individual rows sets an intention exclusive lock on the table.
DDL Locks in InnoDB:
Data Definition Language (DDL) operations require more extensive locks, often at a table level. InnoDB uses metadata locks (MDL) to manage concurrent DDL and DML operations.
- If a transaction is modifying a table (for example, adding a column), that operation requires an exclusive lock, preventing other transactions from modifying data in the table until the operation is complete.
- If a transaction is reading from a table, it can share the read lock with other transactions, but if a DDL operation comes along, it has to wait until the reads are finished.
Internal Locks & Latches in InnoDB:
Internal locks (such as latches and mutexes) are used by InnoDB to protect shared data structures within its code from concurrent access. They are used internally to ensure consistency, and their use is generally invisible to users.
- Mutex (Mutual Exclusion Object): This is a type of lock that protects shared resources. Only one thread can hold a mutex at a time. When a thread holds a mutex, other threads attempting to acquire the same mutex are blocked until the holder releases it.
- Latches: These are lightweight, short-term locks that protect memory structures. They are typically held for short periods of time, such as while a data structure is being inspected or modified.
InnoDB uses a sophisticated lock management system to handle multiple transactions and operations, making sure that the ACID (Atomicity, Consistency, Isolation, Durability) properties of the transactions are maintained. Understanding these locks can help in optimizing database performance and diagnosing issues.