presto-main/src/main/java/io/prestosql/sql/planner/planprinter/PlanPrinter.java/Visitor/getStatisticGroupingSetsInfo
class Visitor:        private String getStatisticGroupingSetsInfo(Map<String, Symbol> columnMappings)
        {
            return columnMappings.entrySet().stream()
                    .map(entry -> format("%s := %s", entry.getValue(), entry.getKey()))
                    .collect(joining(", "));
        }