(The article goes a bit above my head so my excuses if I am a bit off-topic)

There is a form of query plan caching in PG: for prepared statements, if PG determines that the actual value of parameters won't affect the query plan much, it uses a "generic plan" so that it reuses the same query plan for every execution of the prepared statement (https://www.postgresql.org/docs/current/sql-prepare.html, see "notes")

Indeed, and right now it's the only possible way since it remains in a single session, doing otherwise would be very hard.

Unless you count stored procs...

Yes its manual and per session, DB's like MSSQL have that was well but are very rarely used anymore because it got automatic plan caching about 20 years ago which basically eliminates any advantage to manually preparing. Its actually better since it can be shared across all sessions

Honestly I thought the same as you, then I wrote this, and I now understand it's going to be really hard to do. To make it very simple: there are pointers to query parts "leaking" everywhere across the execution engine. Removing them will require a significant overall of the execution engine, the planner and who knows what else. Even in a single session, two compiled queries will have different compiled code because of that (both llvm and my copyjit have to inject the adresses of various structs in asm code)

Just going to say, I'm blown away by how simple this JIT is though. Really quite a beautiful JIT approach.

Same for me, that's why I did this after finding out this research paper. With the proper compiler settings and small tricks you can remove some parts and already end up faster than the interpreter (because you remove some branches and a few memory accesses) and it's even possible to create "super-stencils" covering typical opcodes series and optimizing them further. Or the opposite, "sub-stencils" in order to do some loop unrolling for instance.

The plan cache on Oracle in combination with prepared statements where the optimizer can't peek into the parameters has been really problematic for me in the past. I usually had to go in and either add hints or force a plan.

Even simple queries like SELECT * FROM t WHERE x = TRUE; could turn into a nightmare depending on the distribution of the x values in the table.

With Postgres I rarely encountered such problems but I must admit that I haven't used Postgres with prepared statements.

I have seen some queries with slow planning time (>100ms) where a cache could have been useful but I don't remember ever really needing to optimize one.

Is x a parameter because it doesn't look like it? MSSQL has parameter sniffing and will make multiple plans based on incoming parameters I would be surprised if Oracle does not do the same. It can actually be problematic to sniff parameters sometimes and it can be disabled with a hint!

I just made up something simple but yes, I had these problems with bind variables in prepared statements.

> SELECT * FROM t WHERE x = :var;

But I haven't used Oracle in years and back in the day, there was no bind variable peeking.