That isn't exlicitly clear in Odoo's documentation because the "best" answer often depends on your specific needs and scale.

For small to medium-sized Odoo deployments, it is generally recommended and common practice to have the PostgreSQL database running on the same server as the Odoo application.

Why it's often recommended for smaller setups:

• Simplicity: Easier to set up, manage, and troubleshoot. You have fewer servers to maintain.
• Cost-Effective: Only one server is needed, reducing hosting costs.
• Lower Latency: Odoo communicates with the database over localhost or a very fast internal network, minimizing communication delays. This is often faster than network hops between separate servers.
• Resource Sharing (if balanced): For lighter loads, the server's CPU, RAM, and disk I/O can be efficiently shared between Odoo and PostgreSQL.

What are the factors you should consider to separate the DB into another Server?

Separating the PostgreSQL database onto a dedicated server becomes highly recommended as your Odoo deployment grows in size, complexity, or criticality. Here are the key factors to consider:

1. Performance and Scalability:
   • High User Count: If you have a large number of concurrent Odoo users (e.g., 50+ active users).
   • Large Data Volume: If your database is expected to grow very large (hundreds of GBs or TBs) with extensive historical data, many attachments, or complex module data.
   • Complex Operations: If Odoo is performing very resource-intensive operations frequently (e.g., complex reports, heavy integrations, large data imports/exports).
   • Resource Contention: Odoo and PostgreSQL are both CPU, RAM, and I/O intensive. On a single server, they can compete for these resources, leading to bottlenecks. Separating them allows each to have dedicated hardware resources.
2. Resource Isolation:
   • If you notice that either Odoo or PostgreSQL is consistently maxing out CPU, RAM, or disk I/O on your single server, separating them allows you to scale each component independently. You can give the database server more RAM and faster SSDs, and the Odoo server more CPU cores.
3. Security and Compliance:
   • Enhanced Security: A dedicated database server can be placed in a more restricted network segment, with tighter firewall rules, reducing its exposure to potential threats.
   • Compliance Requirements: Certain industry regulations or compliance standards may mandate physical or logical separation of data layers.
4. Backup and Recovery:
   • Simplified Backups: Backing up a dedicated database server can be more straightforward and efficient, especially for large databases.
   • Point-in-Time Recovery: Easier to implement advanced backup strategies like streaming replication and point-in-time recovery for the database.
5. High Availability (HA) and Disaster Recovery (DR):
   • For mission-critical Odoo instances, you might want to set up PostgreSQL replication (e.g., master-replica) for high availability. This inherently requires at least two database servers.
   • Separation is a prerequisite for robust DR strategies.
6. Management and Specialization:
   • If you have dedicated database administrators (DBAs) or need very specific PostgreSQL tuning, having a dedicated server allows them to manage it independently without affecting the Odoo application server.
7. Cost and Complexity (The Downsides of Separation):
   • Increased Cost: You will need at least two servers (one for Odoo, one for PostgreSQL).
   • Increased Complexity: More network configuration, more points of failure, more components to monitor and manage.
   • Network Latency: While often minimal in a good data center, there will always be some network latency between two separate servers compared to localhost. This is usually negligible unless the network is poorly configured or geographically distant.

Summary of Advice:

• Keep them together (same server): For most small to medium Odoo deployments (e.g., up to 30-50-100 concurrent users, database size under 500-1TB), a single, well-resourced server is often the most efficient and manageable solution.
• Consider separating (dedicated server): When your Odoo instance becomes mission-critical, experiences consistent resource bottlenecks, serves a large number of concurrent users, handles very large data volumes, or if you have specific security/compliance requirements.