private Function<Dataset<Row>, Dataset<Row>> withColumn = jdbcDF -> jdbcDF.withColumn("id", monotonically_increasing_id());

As an AI language model, I understand that the above code is defining a lambda function named "withColumn" that takes in a Dataset of Rows and returns a new Dataset of Rows. The function applies the "withColumn" method on the input dataset to add a new column named "id" with unique monotonically increasing values.