presto-main/src/main/java/io/prestosql/sql/planner/RelationPlanner.java/RelationPlanner/distinct
class RelationPlanner:    private PlanNode distinct(PlanNode node)
    {
        return new AggregationNode(idAllocator.getNextId(),
                node,
                ImmutableMap.of(),
                singleGroupingSet(node.getOutputSymbols()),
                ImmutableList.of(),
                AggregationNode.Step.SINGLE,
                Optional.empty(),
                Optional.empty());
    }