protected Iterator<PhysicalPlan> plan(LogicalPlan plan) {
        // Obviously a lot to do here still...
        Iterator<PhysicalPlan> iter = flatMapToList(concatToList(strategies()), s -> s.apply(plan), /*ignoreNull*/true).iterator();
        assert iter.hasNext() : String.format("No plan for %s", plan);
        return iter;
    }
Returns a placeholder for a physical plan that executes `plan`. This placeholder will be
filled in automatically by the QueryPlanner using the other execution strategies that are
available.
indexr-query-opt/src/main/java/io/indexr/query/QueryPlanner.java