[SPARK-17774] Add support for head on DataFrame Column

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Incomplete
• Affects Version/s: 2.0.0
• Component/s: SparkR
• Labels: bulk-closed
• Assignee: Unassigned
• Reporter: Hossein Falaki
• Created: 04/Oct/16 00:36
• Updated: 21/May/19 04:33
• Resolved: 21/May/19 04:33

Description

There was a lot of discussion on SPARK-9325. To summarize the conversation on that ticket regardign collect

• Pro: Ease of use and maximum compatibility with existing R API
• Con: We do not want to increase maintenance cost by opening arbitrary API. With Spark's DataFrame API collect does not work on Column and there is no need for it to work in R.

This ticket is strictly about head. I propose supporting head on Column because:
1. R users are already used to calling head(iris$Sepal.Length). When they do that on SparkDataFrame they get an error. Not a good experience
2. Adding support for it does not require any change to the backend. It can be trivially done in R code.