That being said, Oracle Database SE2 is $17,500 per core pair on x86, and Enterprise is $47,500 per core pair. XE has hard limits on size and limits on active CPUs. XE also does not get patches; if there is a critical vulnerability, it might be years before an upgrade is released.
Nobody would deploy Oracle Database for new systems. You only use this for sunk costs.
Postgres itself has a manual that is 1,500 pages. There is a LOT to learn to run it well, comparable to Oracle.
For simple things, SQLite is fine. I use it as my secrecy manager.
Postgres requires a lot of reading to do the fancy things.