Kostiantyn Kudriavtsev Thanks for working on this. The code changes seem fine, but I have a couple of high-level questions:
1. The UDF in the patch takes only a date in a string form, which seems too restrictive as Hive has a the data type of Date. Hive should be able to output the date to any format the user specifies. The udf isn't able to achieve this.
2. The name of the UDF, format_date is questionable, because major DBs provide date_format() that does a different thing. This can cause unnecessary confusion.
Thus, to have a bigger usage, I'd suggest the following:
1. provide a UDF, such as to_date(string dateString, String format), that converts a given date in the given format to a Date date. to_date is also available in Oracle, while mysql has STR_TO_DATE.
2. provide UDF, date_format(Date date, String format) as what Oracle or MySQL has.
With these two, you should be able to achieve what you want while retaining bigger flexibility and usage.