private ColumnTransformer getBuiltInScalarFunctionTransformer(
      FunctionExpression expression, ColumnTransformerVisitorContext context) {
    ColumnTransformer childColumnTransformer =
        this.process(expression.getExpressions().get(0), context);
    return BuiltInScalarFunctionHelperFactory.createHelper(expression.getFunctionName())
        .getBuiltInScalarFunctionColumnTransformer(expression, childColumnTransformer);
  }
Responsible for constructing {@link ColumnTransformer} through Expression.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/visitor/ColumnTransformerVisitor.java