This is the thing I love about PostgreSQL, you can find out everything about the database. Oracle is ridiculous - they have something of the range of over a thousand hidden parameters, query tuning their CBO is somewhat of a black art and can change between point releases, and leads to articles like the following:
Oracle also has serious quality issues. The last patch set I applied had the wrong binaries for Oracle Text. With what they charge, you wouldn't expect that. I've never had such a thing happen with my Postgresql instances. I just don't see how to justify starting out with Oracle today.
Don’t take anything seriously on dba-oracle.com - Burleson is well known in the Oracle community as an expert in self-promotion but his technical skill is mediocre at best.
Jonathan Lewis, Guy Harrison and Craig “the Hammer” Shallahammer are infinitely more experienced and insightful.
Right. There are many Oracle internals experts who are presenting at conferences, they are also quite approachable. I'd especially recommend Tanel Poder, several of his troubleshooting sessions are available on youtube.
That's sometimes unavoidable. If there's a clear bug somewhere, fixing it will sometimes also affect queries/plans that weren't visibly affected by the bug. That's especially the case around costing issues - some queries might have regressed noticeably due to a bug since the last major version, but fixing it might affect other queries negatively (in a minor version). Postgres/we try to avoid that, but sometimes that's the most sensible way forward (and I assume the same is true for mysql etc).
http://www.dba-oracle.com/art_so_undoc_parms_p2.htm
PostgreSQL, however, has none of these limitations.