presto-main/src/main/java/io/prestosql/sql/planner/optimizations/joins/JoinGraph.java/Builder/visitPlan
class Builder:        @Override
        protected JoinGraph visitPlan(PlanNode node, Context context)
        {
            for (Symbol symbol : node.getOutputSymbols()) {
                context.setSymbolSource(symbol, node);
            }
            return new JoinGraph(node);
        }