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What is Write-Ahead Log (WAL)?

A technique where changes are first written to a log before being applied to the database, ensuring crash recovery and data integrity.

Write-Ahead Logging is fundamental to database reliability. Before any data modification is written to the actual data files, it is first recorded in a sequential log. If the database crashes, it can replay the WAL to recover committed transactions and undo incomplete ones. PostgreSQL uses WAL extensively — it powers crash recovery, point-in-time recovery (PITR), streaming replication, and logical replication. WAL settings like wal_level, max_wal_size, and checkpoint_timeout directly impact performance and recovery capabilities.

Related Terms

Connection String
A formatted string containing all parameters needed to establish a connection to a database server.
Transaction
A sequence of database operations that are treated as a single unit — either all succeed or all are rolled back.
JOIN
An SQL operation that combines rows from two or more tables based on a related column between them.
Deadlock
A situation where two or more transactions permanently block each other by each holding locks that the other needs.
CTE (Common Table Expression)
A temporary named result set defined within a SQL statement using the WITH clause, improving query readability and enabling recursion.
Database Backup
The process of creating copies of database data to protect against data loss from failures, corruption, or human error.
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