Enumeration is non-deterministic

[juripetersen]

Using Wayangs default enumeration and pruning strategies, the plans compared in cost models are non-deterministic.
This can be observed when comparing costs with different mechanisms:

            final PlanImplementation bestPlanImplementation = executionPlans.stream()
                .reduce((p1, p2) -> {
                    final double t1 = p1.getSquashedCostEstimate();
                    final double t2 = p2.getSquashedCostEstimate();
                    return t1 < t2 ? p1 : p2;
                })

[novatechflow]

Ya stumbled upon as I tested #646 - think about to replace the HashSet usage in PlanEnumeration/MultiMap with LinkedHashSet, or convert the sets into deterministically sorted Lists (e.g., order by operator IDs) before feeding to the collector.

[juripetersen]

We already did exactly this in a fork of ours. However, that didn't solve the non-deterministic nature.

[novatechflow]

Yeah, there are a lot of unordered collections across the channels, I saw the helper keeps rehashing, that's the main issue I think. Side effects then unordered collections, which let the graph go havoc. I write a regression test today to find other places.