Re: User's responsibility when using a chain of "immutable" functions?

Meanwhile. I'll appeal for some pointers to what I should read:

I *had* understood that the SQL that a user-created subprogram issues (at least for "language sql" and "language plpgsql") is implicitly prepared. But I've no idea what it uses as the "handle" for such a prepared statement. Might it be, for example, a node in the AST that represents the subprogram or anonymous block in my session? In the same way, I've no idea what the outcome is when two different subprograms issue the identical (or identical post-canonicalization) SQL statement text. I don't know how to search the PG doc to find the explanations that I need. For example "pl/pgsql execution model" gets just a single hit in in a piece about locking.

Wider Internet search gets too much noise, and too much unreliable suff from self-appointed experts, to be of much help. I was excited to find "Plpgsql_internals.pdf" by pavel.stehule@gmail.com. But disappointed to find that it didn't answer my questions (and nor could it when it explains things w.r.t the C implementation).

Does anybody have any recommendations for what I might study?

Of course, I started with the account of "immutable" in the "create function" doc:

«
IMMUTABLE indicates that the function cannot modify the database and always returns the same result when given the same argument values; that is, it does not do database lookups or otherwise use information not directly present in its argument list. If this option is given, any call of the function with all-constant arguments can be immediately replaced with the function value.

»

"immediately replaced with the function value" implies a cache. But not when it's populated (and when it isn't) or what its lifetime might be. It certainly doesn't mention cache invalidation.

/*
  Presumably dropping f1() invalidates q1 but
  leaves its definition intact so that it can later be re-vaildated
  when f1() exists again.
*/;
drop function f1() cascade;
execute q2; --------------------------------<< Still gets "dog"

--------------------------------------------------------------------------------

-- TEST TWO. BREAKS MY MENTAL MODEL.

-- ALL I DID WAS CHANGE f1() AND f2() TO HAVE A TEXT PARAMETER

-- Now fails with "function f1(text) does not exist"
execute q2('CAT');