It's been many years since I've had to use an Oracle db, but they definitely allowed SQL developers to forcibly specify a query-plan - called "query hints", wherein you could specify it to prefer using (or avoiding) certain indexes, certain join-strategies (hash, loop, etc) - this was done via comments immediately before the SQL query - see https://docs.oracle.com/cd/B13789_01/server.101/b10752/hints...
Postgresql doesn't have this, however, I've rarely missed this feature - tables with good indexes, regular db hygiene, etc, almost always perform excellently. The query planner is very, very smart nowadays.
The query hints thing is actually quite useful in an educational context.
There's DB internals/performance course which I'm a TA for and we lean heavily on being able to force Oracle's rule-based or cost-based optimisers --- because then we can get the students to analyse and compare the plans.
Postgresql doesn't have this, however, I've rarely missed this feature - tables with good indexes, regular db hygiene, etc, almost always perform excellently. The query planner is very, very smart nowadays.