presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/LocalExecutionPlanner/visitSample
class LocalExecutionPlanner:        @Override
        public PhysicalOperation visitSample(SampleNode node, LocalExecutionPlanContext context)
        {
            // For system sample, the splits are already filtered out, so no specific action needs to be taken here
            if (node.getSampleType() == SampleNode.Type.SYSTEM) {
                return node.getSource().accept(this, context);
            }

            throw new UnsupportedOperationException("not yet implemented: " + node);
        }