presto-memory/src/main/java/io/prestosql/plugin/memory/MemoryDataFragment.java/MemoryDataFragment/merge
class MemoryDataFragment:    public static MemoryDataFragment merge(MemoryDataFragment a, MemoryDataFragment b)
    {
        checkArgument(a.getHostAddress().equals(b.getHostAddress()), "Can not merge fragments from different hosts");
        return new MemoryDataFragment(a.getHostAddress(), a.getRows() + b.getRows());
    }