Understanding the Internal Locking Hierarchy and Mechanisms in PostgreSQL

In PostgreSQL, the locking mechanism is crucial for concurrency control, ensuring data integrity and consistency by preventing simultaneous incompatible operations. PostgreSQL employs a variety of lock types that operate at different levels of granularity, from entire databases down to individual rows. The locking hierarchy in PostgreSQL can be understood in terms of the scope and granularity of the locks, which includes database locks, table-level locks, page locks, and tuple (row) locks, among others.

Lock Types and Hierarchy

  1. Database Locks: These locks are used for operations that affect the whole database, such as DROP DATABASE.
  2. Table-level Locks: These are used for operations affecting entire tables. Table-level locks come in different modes, like ACCESS SHARE (used for SELECT operations), ROW SHARE (used for UPDATE, DELETE), ACCESS EXCLUSIVE (used for operations like DROP TABLE, TRUNCATE, which block all other operations), and several others. Each lock mode determines the compatibility with other lock modes.
  3. Row-level Locks: PostgreSQL implements row-level locking in a way that is mostly transparent to the user, using two mechanisms: lock flags within the table’s hidden xmin and xmax system columns for each row (used for MVCC), and explicit row locks (SELECT FOR UPDATE, for example). Row-level locks offer the finest granularity, allowing high concurrency.
  4. Advisory Locks: These are locks that are not tied to a specific database object but are used by applications to achieve synchronization between sessions for application-level operations.

Lock Modes and Compatibility

PostgreSQL defines various lock modes, which determine whether different operations are compatible with each other. For example, multiple transactions can hold SHARE UPDATE EXCLUSIVE locks on a table simultaneously, but an ACCESS EXCLUSIVE lock is incompatible with any other lock, effectively serializing access to the resource.

Deadlocks

PostgreSQL automatically detects deadlocks, situations where two or more transactions are waiting for each other to release locks. When detected, PostgreSQL will abort one of the transactions to break the cycle, allowing the other transactions to proceed.

Internal Locking Mechanisms

Internally, PostgreSQL uses a lock table to manage most types of locks. The lock table maps lockable objects to the list of transactions holding or waiting for locks on them. For row-level locks, PostgreSQL uses a combination of predicate locks for Serializable transactions and lightweight locks or flags directly on rows for other isolation levels, minimizing overhead and maximizing performance.

Managing Locks

PostgreSQL administrators can view lock information using system views like pg_locks, pg_class, and pg_stat_activity. These views can be queried to analyze current locks, which sessions are holding them, and potential locking issues.

Conclusion

Understanding the locking hierarchy and behavior in PostgreSQL is essential for database administration, performance tuning, and application development. Proper use of locks ensures data integrity and consistency while maximizing concurrency. Administrators and developers should design database operations with an understanding of lock compatibility and granularity to avoid unnecessary locking and potential performance issues.

About Shiv Iyer 444 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.