presto-orc/src/main/java/io/prestosql/orc/reader/MapColumnReader.java/MapColumnReader/getRetainedSizeInBytes
class MapColumnReader:    @Override
    public long getRetainedSizeInBytes()
    {
        return INSTANCE_SIZE + keyColumnReader.getRetainedSizeInBytes() + valueColumnReader.getRetainedSizeInBytes();
    }