@Override
  public void evaluateWithSelection(boolean[] selection) {
    ColumnBuilder builder = returnType.createColumnBuilder(selection.length);
    Column inputColumn = input.getColumn(inputIndex);
    for (int i = 0; i < selection.length; i++) {
      if (!selection[i] || inputColumn.isNull(i)) {
        builder.appendNull();
      } else {
        builder.write(inputColumn, i);
      }
    }
    initializeColumnCache(builder.build());
  }
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.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/column/leaf/IdentityColumnTransformer.java