[SPARK-21897][PYTHON][R] Add unionByName API to DataFrame in Python and R

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to add a wrapper for unionByName API to R and Python as well.

Python

df1 = spark.createDataFrame([[1, 2, 3]], ["col0", "col1", "col2"])
df2 = spark.createDataFrame([[4, 5, 6]], ["col1", "col2", "col0"])
df1.unionByName(df2).show()

+----+----+----+
|col0|col1|col3|
+----+----+----+
|   1|   2|   3|
|   6|   4|   5|
+----+----+----+

R

df1 <- select(createDataFrame(mtcars), "carb", "am", "gear")
df2 <- select(createDataFrame(mtcars), "am", "gear", "carb")
head(unionByName(limit(df1, 2), limit(df2, 2)))

  carb am gear
1    4  1    4
2    4  1    4
3    4  1    4
4    4  1    4

How was this patch tested?

Doctests for Python and unit test added in test_sparkSQL.R for R.

[HyukjinKwon]

cc @felixcheung and @actuaryzhang, could you take a look please?

[SparkQA]

Test build #81335 has finished for PR 19105 at commit 058feeb.

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

[viirya]

Python side change LGTM

[felixcheung]

do you mean for your example in the PR description above something different?
it says

head(union(limit(df1, 2), limit(df2, 2)))

[felixcheung]

LGTM, minor comments

[felixcheung]

I think I'd suggest a slight different in the title - this one is the same words as union

[HyukjinKwon]

Sure, I just addressed both comments.

[felixcheung]

I'd list union() here too

[HyukjinKwon]

Oh, yea. I made a mistake during copying and pasting the example in the description. I just corrected it.

[SparkQA]

Test build #81341 has finished for PR 19105 at commit 7ce6713.

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

[HyukjinKwon]

Thank you @viirya and @felixcheung.

[HyukjinKwon]

Merged to master.