Postgres WAL Log vs Logical Replication
Postgres WAL Log vs Postgres Logical Replication – When to Use Which?
In the world of PostgreSQL administration, understanding the difference between Postgres WAL log and Postgres logical replication is critical for designing high availability, disaster recovery, and data distribution strategies.
If you’re serious about building a career in database administration, understanding replication is non-negotiable. In our PostgreSQL DBA online training, one of the most important topics we cover is the difference between Postgres WAL log and Postgres logical replication.
Many DBAs confuse these two technologies because both move data from one server to another. However, their purpose, architecture, and use cases are very different.
In this guide, we will break down:
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What is Postgres WAL log?
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What is Postgres logical replication?
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Key differences
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Real-world use cases
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When a PostgreSQL DBA should choose one over the other
What is Postgres WAL Log?
The Postgres WAL log (Write-Ahead Log) is the backbone of PostgreSQL’s durability and crash recovery mechanism.
Whenever a change is made in PostgreSQL (INSERT, UPDATE, DELETE), it is first written to the WAL log before being written to data files.
Why WAL Exists
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Ensures ACID compliance
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Enables crash recovery
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Supports physical replication
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Enables Point-in-Time Recovery (PITR)
In physical streaming replication, standby servers continuously receive and replay WAL logs to stay in sync with the primary server.
What is Postgres Logical Replication?
Postgres logical replication replicates data at the logical level (tables, rows, DML operations).
Instead of replaying raw WAL blocks, logical replication sends decoded changes like:
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INSERT statements
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UPDATE statements
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DELETE statements
Logical replication allows selective replication of specific tables instead of the entire database cluster.
Key Differences: Postgres WAL Log vs Postgres Logical Replication
| Feature | Postgres WAL Log (Physical Replication) | Postgres Logical Replication |
|---|---|---|
| Level | Physical (block-level) | Logical (row-level) |
| Replicates | Entire cluster | Selected tables |
| Version Compatibility | Same major version required | Can replicate across versions |
| Read-Only Standby | Yes | Subscriber is writable |
| Use Case | HA, Disaster Recovery | Data sharing, migrations |
| Performance | Faster | Slightly slower |
When Should a PostgreSQL DBA Use WAL-Based Replication?
Choose Postgres WAL log replication when:
- need High Availability
- want disaster recovery
- need exact physical copy of primary
- want minimal replication overhead
- need Point-in-Time Recovery
This is ideal for production HA clusters.
When Should a PostgreSQL DBA Use Logical Replication?
Choose Postgres logical replication when:
- want to replicate selected tables
- migrating between versions
- need multi-master-like behavior
- want reporting servers with custom data
- need cross-region data distribution
Logical replication is flexible and powerful for modern architectures.
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