@Override
  protected Column[] transform(Column[] columns) throws QueryProcessException, IOException {
    return columns;
  }
this is a special transformer which outputs data just as input without any modification.

<p>i.e. it's just the function f(x) = x.

<p>It's mainly used for a UDF with aggregation query as its parameters.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/transformer/unary/TransparentTransformer.java