@Override
  public ColumnTransformer visitTimeSeriesOperand(
      TimeSeriesOperand timeSeriesOperand, ColumnTransformerVisitorContext context) {
    ColumnTransformer res =
        context.cache.computeIfAbsent(
            timeSeriesOperand,
            e -> {
              IdentityColumnTransformer identity =
                  new IdentityColumnTransformer(
                      TypeFactory.getType(context.getType(timeSeriesOperand)),
                      context
                          .inputLocations
                          .get(timeSeriesOperand.getExpressionString())
                          .get(0)
                          .getValueColumnIndex());
              context.leafList.add(identity);
              return identity;
            });
    res.addReferenceCount();
    return res;
  }
Responsible for constructing {@link ColumnTransformer} through Expression.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/visitor/ColumnTransformerVisitor.java