presto-main/src/main/java/io/prestosql/operator/TableScanOperator.java/TableScanOperator/finish
class TableScanOperator:    @Override
    public void finish()
    {
        finished = true;
        blocked.set(null);

        if (source != null) {
            try {
                source.close();
            }
            catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            systemMemoryContext.setBytes(source.getSystemMemoryUsage());
        }
    }