Locking in PostgreSQL is a mechanism used to control concurrent access to shared resources, ensuring data consistency and preventing conflicting operations. PostgreSQL offers various lock types, each serving a specific purpose. Let’s explore how locking works in PostgreSQL.
Lock Modes
PostgreSQL supports different lock modes, including:
Access Share (SELECT): Allows concurrent read-only access to a resource.
Row Share (SELECT FOR UPDATE): Allows read access and allows other sessions to read the same resource.
Row Exclusive (UPDATE, DELETE): Allows exclusive access to a specific row, preventing concurrent writes or deletes on the same row.
Share Update Exclusive (VACUUM): Allows read access and exclusive write access to the entire table, but not to individual rows.
Share (CREATE INDEX): Allows concurrent read access and concurrent index creation on a table.
Share Row Exclusive (ALTER TABLE): Allows concurrent read access and schema modifications on a table.
Exclusive (ALTER TABLE, DROP TABLE): Provides exclusive access to a resource, preventing any other concurrent access.
Access Exclusive (CREATE/DROP DATABASE): Allows exclusive access to a database.
Lock Granularity
PostgreSQL supports various levels of lock granularity, allowing locks to be acquired at different levels:
Row-Level Locks: Provide the finest level of granularity, allowing locks to be acquired on specific rows within a table. Multiple sessions can concurrently hold row-level locks on different rows of the same table.
Page-Level Locks: Locks are acquired at the page level, which is a group of database blocks. Multiple sessions can hold page-level locks on different pages within the same table.
Table-Level Locks: Locks are acquired on the entire table, preventing concurrent access to any rows or pages within the table.
Lock Compatibility
PostgreSQL uses a lock compatibility matrix to determine which locks can coexist and which conflict with each other. The compatibility matrix ensures that transactions do not interfere with each other by acquiring incompatible locks simultaneously.
For example, an exclusive lock on a row conflicts with other exclusive or shared locks on the same row but is compatible with an access share lock.
Locking and Transaction Isolation Levels:
Transaction isolation levels in PostgreSQL determine the degree to which concurrent transactions can interact with each other. The isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) influence the behavior of locking and concurrency control.
Read Uncommitted: No locks are acquired, allowing dirty reads and potential data inconsistencies.
Read Committed: Exclusive locks are acquired for write operations, preventing dirty reads and ensuring data consistency within a transaction.
Repeatable Read: Additional locks are acquired to ensure that data read within a transaction remains consistent, preventing phantom reads.
Serializable: Provides the strictest isolation level, ensuring that concurrent transactions do not produce non-repeatable reads, phantom reads, or serialization anomalies. This level acquires more locks to enforce serializability.
Lock Timeout and Deadlocks
PostgreSQL allows you to set a timeout for acquiring locks. If a lock cannot be acquired within the specified timeout, an error is raised. Deadlocks can occur when two or more transactions are waiting for locks held by each other, leading to a circular dependency. PostgreSQL employs a deadlock detection mechanism that detects such situations and aborts one of the transactions involved to resolve the deadlock.
Conclusion
It is essential to understand how locking works in PostgreSQL to optimize transaction management, prevent contention, and ensure efficient concurrency control. By choosing the appropriate lock modes, understanding lock compatibility, and setting the right transaction isolation level, you can design robust and performant applications on PostgreSQL.
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