@Override
  public ColumnTransformer visitConstantOperand(
      ConstantOperand constantOperand, ColumnTransformerVisitorContext context) {
    ColumnTransformer res =
        context.cache.computeIfAbsent(
            constantOperand,
            e -> {
              ConstantColumnTransformer columnTransformer =
                  new ConstantColumnTransformer(
                      TypeFactory.getType(context.getType(constantOperand)),
                      TransformUtils.transformConstantOperandToColumn(constantOperand));
              context.leafList.add(columnTransformer);
              return columnTransformer;
            });
    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