what does #function[X/Y/fn--10200] mean?

Is that like a function applied to arguments?

#function is a reader literal that tells the reader how to process the form that follows it.

X/Y/fn--10200 is the compiled symbol name of, I think, an anonymous function in a namespace X.Y...

So the meaning depends on the code that is producing that and reading it.

What's the context for that @henri.schmidt?

Are you talking about this sort of output

user=> (defn f [] (fn [] 42))
#'user/f
user=> (f)
#object[user$f$fn__149 0x1eaf1e62 "user$f$fn__149@1eaf1e62"]
user=>                                                        

I feel like once you learn about Datomic, and how you can query for values over time, it’s frustrating that this feels like such a left-out feature of other relational DBs