presto-main/src/main/java/io/prestosql/sql/planner/RelationPlanner.java/RelationPlanner/visitExcept
class RelationPlanner:    @Override
    protected RelationPlan visitExcept(Except node, Void context)
    {
        checkArgument(!node.getRelations().isEmpty(), "No relations specified for EXCEPT");

        SetOperationPlan setOperationPlan = process(node);

        PlanNode planNode = new ExceptNode(idAllocator.getNextId(), setOperationPlan.getSources(), setOperationPlan.getSymbolMapping(), ImmutableList.copyOf(setOperationPlan.getSymbolMapping().keySet()));
        return new RelationPlan(planNode, analysis.getScope(node), planNode.getOutputSymbols());
    }