presto-hive/src/main/java/io/prestosql/plugin/hive/HivePageSource.java/HivePageSource/load
class HivePageSource:        @Override
        public Block load()
        {
            checkState(block != null, "Already loaded");

            Block loaded = coercer.apply(block.getLoadedBlock());
            // clear reference to loader to free resources, since load was successful
            block = null;

            return loaded;
        }