if one tests a potential schema and profiles some queries with test data using dev-local do the results predict real-world performance once deployed to AWS ?

No

the query engine will work the same way but almost certainly the cardinalities of your entities are smaller in your tests, as well as differing in storage and caching characteristics

given enough realistic sample data, you can use the newly released https://blog.datomic.com/2023/01/Query-Stats.html functionality to gain confidence

basically what joe said, no

ok thank you

but does local query performance vs cardinality not even scale similarly to that on AWS ?

and surely a change in :where order which resulted in better performance of a query locally would also result in better performance on AWS ?

yes the query execution plans will translate, it's the same engine

storage stack is very different though

yes I get that

does the query planner consider cardinality when constructing its execution plan?

Datomic executes your :where clauses in the order you specify: https://docs.datomic.com/on-prem/best-practices.html#most-selective-clauses-first That said, with the addition of https://docs.datomic.com/on-prem/api/query-stats.html, you can now observe per-clause cardinalities and adjust your clause ordering accordingly.

is grounding a unique value enough to bust the cache when running a query?

'{:where [[?item :item/id 123]
          [(ground "some-unique-value") ?grounded]]}

With the new query-stats feature (which is great!) I’ve been able to observe that it doesn’t always execute the :where clauses in the specified order. Here is a real example from a debugging session yesterday. Note that the [(!= ?group ?group2)] clause has been moved up in the :sched plan compared to its location in the original :where clause. I’m just wondering if that reordering is predictable/stable/consistent or if it depends on the cardinality of the data in some way.

:query-stats
 {:query
  {:find [[?group2 ...]],
   :in [$ ?group],
   :where
   [(not [?group :group/archived?])
    [?group :group/workspace2 ?wksp2]
    [?group :group/name ?name]
    [?group2 :group/workspace2 ?wksp2]
    (not [?group2 :group/archived?])
    [?group2 :group/name ?name]
    [(!= ?group ?group2)]]},
  :phases
  [{:sched
    (([(ground $__in__2) ?group]
      (not-join [?group] [?group :group/archived?])
      [?group :group/workspace2 ?wksp2]
      [?group :group/name ?name]
      [?group2 :group/workspace2 ?wksp2]
      [(!= ?group ?group2)]
      (not-join [?group2] [?group2 :group/archived?])
      [?group2 :group/name ?name])),