What complex aspects have you encountered when working with PostgreSQL?

When working with PostgreSQL, the following complex aspects are often encountered:

• Query and index optimization for large data volumes to avoid long delays.
• Transaction and lock management, especially under high concurrency, to prevent deadlocks.
• Configuration and use of advanced features like JSONB, full-text search, and custom data types.
• Replication and backup for fault tolerance.
• Schema migrations and maintaining compatibility.

For example, when using JSONB, it is important to index fields properly for query efficiency, and when working with transactions, to choose the right isolation level to avoid concurrency issues.