presto-main/src/main/java/io/prestosql/operator/DeleteOperator.java/DeleteOperator/pageSource
class DeleteOperator:    private UpdatablePageSource pageSource()
    {
        Optional<UpdatablePageSource> source = pageSource.get();
        checkState(source.isPresent(), "UpdatablePageSource not set");
        return source.get();
    }