presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/NodePrinter/visitIndexSource
class NodePrinter:        @Override
        public Void visitIndexSource(IndexSourceNode node, Void context)
        {
            printNode(node, format("IndexSource[%s]", node.getIndexHandle()), NODE_COLORS.get(NodeType.INDEX_SOURCE));
            return null;
        }