presto-main/src/main/java/io/prestosql/sql/planner/iterative/Memo.java/Memo/Memo
class Memo:    public Memo(PlanNodeIdAllocator idAllocator, PlanNode plan)
    {
        this.idAllocator = idAllocator;
        rootGroup = insertRecursive(plan);
        groups.get(rootGroup).incomingReferences.add(ROOT_GROUP_REF);
    }