How to append column header to Spark SQL query results without using databricks lib?

[#1]

Hi All, How can we add a header to Spark SQL Query results before saving the results in a textfile? Spark version is 1.6

    val topPriceResultsDF = sqlContext.sql("SELECT * FROM retail_db.yahoo_stock_orc WHERE open_price > 40 AND high_price > 40 ORDER BY date ASC")
    topPriceResultsDF.map(x => x.mkString(",")).saveAsTextFile("/user/sparkuser/myspark/data/output/yahoo_above40_results(comma).csv") 

It saves only data but I need to add header like (date,open_price,high_price,low_price,close_price,volume,adj_price) as well . Please help if anyone has idea !! I cannot use databricks library.

O/P should be like 
    date,open_price,high_price,low_price,close_price,volume,adj_price
    1997-07-09,40.75008,45.12504,40.75008,43.99992,37545600,1.83333 

Thanks !!

[#2]

Use mapPartitions if we want to add header in all files or if there is single partition.

topPriceResultsDF
.map(x => x.mkString(","))
.mapPartitions(iter => Iterator(header) ++ iter)
.saveAsTextFile("/user/sparkuser/myspark/data/output/yahoo_above40resultsWithHeader.csv")

Use mapPartitionsWithIndex if we want to add header in only first file

topPriceResultsDF.map(x => x.mkString(","))
.repartition(2)
.mapPartitionsWithIndex ({
case (0, iter) => Iterator(header) ++ iter
case (_, iter) => iter
})
.saveAsTextFile("/user/sparkuser/myspark/data/output/yahoo_above40resultsWithHeader.csv")