[SPARK-9317] [SPARKR] Change show to print DataFrame entries

[felixcheung]

Small update to DataFrame API in SparkR

@shivaram

[shivaram]

Jenkins, ok to test

[SparkQA]

Test build #41375 has finished for PR 8360 at commit e3ae104.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[davies]

@felixcheung show() will be called when the result of an expression is a DataFrame, so show() should be cheap. But showDF() is not, especially when you have a large dataset and multiple stages (groupBy or join).

[shivaram]

Hmm I can see both sides of this. The problem is that R users are used to seeing the first few rows of a DataFrame if you just print the name out and this will restore that behavior

cc @rxin

[felixcheung]

From what I can infer from the original JIRA is that we are trying to match R data.frame behavior.
I think it is handy, though it is easy to think of several alternative ways to do this (head(df), showDF(df)) but those will need to be learned.

[shivaram]

@rxin Any thoughts on this ? If we want to change show it might be a good idea to grab this for 1.5

[rxin]

Unfortunately I don't think we should make this change right now.

We can however make a change in the future to automatically show if the dataframe is local (or small). I've been thinking about that -- it'd be great to do that for both Python, Scala, and R.

[shivaram]

Alright I'll update the JIRA and since I guess we want this for all languages I'll mark it as a SQL issue as well.

@felixcheung I think we can close this PR for now and revisit this when we have support from the Scala backend for this.

[felixcheung]

ok, closing. let me know what I can help with!