presto-main/src/main/java/io/prestosql/sql/planner/optimizations/joins/JoinGraph.java/Edge/Edge
class Edge:        public Edge(PlanNode targetNode, Symbol sourceSymbol, Symbol targetSymbol)
        {
            this.targetNode = requireNonNull(targetNode, "targetNode is null");
            this.sourceSymbol = requireNonNull(sourceSymbol, "sourceSymbol is null");
            this.targetSymbol = requireNonNull(targetSymbol, "targetSymbol is null");
        }