@Override
  protected void evaluate() {
    initializeColumnCache(input.getColumn(inputIndex));
  }
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