Someone wanted to learn this too, so Grasp built them a personal learning path.
You can keep exploring this one or create your own
PostgreSQL
Module 1
PostgreSQL Architecture and Working Environment
Module 2
Tables, Types, Constraints, and Data Changes
Module 3
Retrieving and Combining Relational Data
Module 4
Relational Design and Reusable Queries
Module 5
PostgreSQL SQL for Application Workloads
Module 6
Transactions, Concurrency, and Storage Internals
Module 7
Indexes and Query Performance
Module 8
Administration, Security, and Recovery
Module 9
Python Application Integration
Module 10
PostGIS Data Models and Coordinate Systems
Module 11
Spatial Queries, Analysis, and Mapping
Module 12