I think the point is that there is no clear reason System A must do more work than System B. Serious Datalog environments will identify strictly monotonic programs and execute them more efficiently (e.g. with racy asynchrony); they will identify linear recursion and partition work accordingly. They could identify programs corresponding to DannyBee's hypothetical purpose-build industrial winners and execute them using the exact same techniques, but apparently they don't.

I suspect the point of disagreement is

> Of course you might wish for a generic Datalog solver that you could run in a special mode where you promise that it will not encounter negation in the problem you give it. But that is then no longer a Datalog solver but a "well engineered purpose built solver".

which I don't think is correct. A Datalog solver with the property that it can run faster on certain classes of input is great; it is not "no longer a Datalog solver".