Error in the code sample of the `implicits` object in `SQLContext`.

Author: Lomig Mégard <lomig.megard@gmail.com>

Closes #5051 from tarfaa/simple and squashes the following commits:

5a88acc [Lomig Mégard] [docs][minor] Fixed sample code in SQLContext scaladoc

[SPARK-6299][CORE] ClassNotFoundException in standalone mode when running groupByKey with class defined in REPL

```
case class ClassA(value: String)
val rdd = sc.parallelize(List(("k1", ClassA("v1")), ("k1", ClassA("v2")) ))
rdd.groupByKey.collect
```
This code used to be throw exception in spark-shell, because while shuffling ```JavaSerializer```uses ```defaultClassLoader``` which was defined like ```env.serializer.setDefaultClassLoader(urlClassLoader)```.

It should be ```env.serializer.setDefaultClassLoader(replClassLoader)```, like
```
    override def run() {
      val deserializeStartTime = System.currentTimeMillis()
      Thread.currentThread.setContextClassLoader(replClassLoader)
```
in TaskRunner.

When ```replClassLoader``` cannot be defined, it's identical with ```urlClassLoader```

Author: Kevin (Sangwoo) Kim <sangwookim.me@gmail.com>

Closes #5046 from swkimme/master and squashes the following commits:

fa2b9ee [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
6e9620b [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
d23e4e2 [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
a4a3c8a [Kevin (Sangwoo) Kim] add 'class defined in repl - shuffle' test to ReplSuite
bd00da5 [Kevin (Sangwoo) Kim] add 'class defined in repl - shuffle' test to ReplSuite
c1b1fc7 [Kevin (Sangwoo) Kim] use REPL class loader for executor's serializer

---- comment;

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #4500 from adrian-wang/semicolon and squashes the following commits:

70b8abb [Daoyuan Wang] use mkstring instead of reduce
2d49738 [Daoyuan Wang] remove outdated golden file
317346e [Daoyuan Wang] only skip comment with semicolon at end of line, to avoid golden file outdated
d3ae01e [Daoyuan Wang] fix error
a11602d [Daoyuan Wang] fix comment with semicolon at end

SparkSubmit should be launched without setting PYSPARK_SUBMIT_ARGS

cc JoshRosen , this mode is actually used by python unit test, so I will not add more test for it.

Author: Davies Liu <davies@databricks.com>

Closes #5019 from davies/fix_submit and squashes the following commits:

2c20b0c [Davies Liu] fix launch spark-submit from python

Currently we would create a new streaming tab for each streamingContext even if there's already one on the same sparkContext which would cause duplicate StreamingTab created and none of them is taking effect.
snapshot: https://www.dropbox.com/s/t4gd6hqyqo0nivz/bad%20multiple%20streamings.png?dl=0
How to reproduce:
1)
import org.apache.spark.SparkConf
import org.apache.spark.streaming.
{Seconds, StreamingContext}
import org.apache.spark.storage.StorageLevel
val ssc = new StreamingContext(sc, Seconds(1))
val lines = ssc.socketTextStream("localhost", 9999, StorageLevel.MEMORY_AND_DISK_SER)
val words = lines.flatMap(_.split(" "))
val wordCounts = words.map(x => (x, 1)).reduceByKey(_ + _)
wordCounts.print()
ssc.start()
.....
2)
ssc.stop(false)
val ssc = new StreamingContext(sc, Seconds(1))
val lines = ssc.socketTextStream("localhost", 9999, StorageLevel.MEMORY_AND_DISK_SER)
val words = lines.flatMap(_.split(" "))
val wordCounts = words.map(x => (x, 1)).reduceByKey(_ + _)
wordCounts.print()
ssc.start()

Author: lisurprise <zhichao.li@intel.com>

Closes #4828 from zhichao-li/master and squashes the following commits:

c329806 [lisurprise] add test for attaching/detaching streaming tab
51e6c7f [lisurprise] move detach method into StreamingTab
31a44fa [lisurprise] add unit test for attaching and detaching new tab
db25ed2 [lisurprise] clean code
8281bcb [lisurprise] clean code
193c542 [lisurprise] remove streaming tab while closing streaming context

If I'm running this locally and my path points to S3, this would currently error out because of incorrect FS.
I tested this in a scenario that previously didn't work, this change seemed to fix the issue.

Author: Volodymyr Lyubinets <vlyubin@gmail.com>

Closes #5020 from vlyubin/parquertbug and squashes the following commits:

a645ad5 [Volodymyr Lyubinets] Fix filesystem bug in newParquet relation

Still, we keep only a single HiveContext within ThriftServer, and we also create a object called `SQLSession` for isolating the different user states.

Developers can obtain/release a new user session via `openSession` and `closeSession`, and `SQLContext` and `HiveContext` will also provide a default session if no `openSession` called, for backward-compatibility.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #4885 from chenghao-intel/multisessions_singlecontext and squashes the following commits:

1c47b2a [Cheng Hao] rename the tss => tlSession
815b27a [Cheng Hao] code style issue
57e3fa0 [Cheng Hao] openSession is not compatible between Hive0.12 & 0.13.1
4665b0d [Cheng Hao] thriftservice with single context

when i run cmd like that sc.addFile("../test.txt"), it did not work and throwed an exception:
java.lang.IllegalArgumentException: java.net.URISyntaxException: Relative path in absolute URI: file:../test.txt
at org.apache.hadoop.fs.Path.initialize(Path.java:206)
at org.apache.hadoop.fs.Path.<init>(Path.java:172)
........
.......
Caused by: java.net.URISyntaxException: Relative path in absolute URI: file:../test.txt
at java.net.URI.checkPath(URI.java:1804)
at java.net.URI.<init>(URI.java:752)
at org.apache.hadoop.fs.Path.initialize(Path.java:203)

Author: DoingDone9 <799203320@qq.com>

Closes #4993 from DoingDone9/relativePath and squashes the following commits:

ee375cd [DoingDone9] Update SparkContextSuite.scala
d594e16 [DoingDone9] Update SparkContext.scala
0ff3fa8 [DoingDone9] test for add file
dced8eb [DoingDone9] Update SparkContext.scala
e4a13fe [DoingDone9] getCanonicalPath
161cae3 [DoingDone9] Merge pull request #4 from apache/master
c87e8b6 [DoingDone9] Merge pull request #3 from apache/master
cb1852d [DoingDone9] Merge pull request #2 from apache/master
c3f046f [DoingDone9] Merge pull request #1 from apache/master

Changed method invocation of 'diff' to match that of 'innerJoin' and 'leftJoin' from VertexRDD[VD] to RDD[(VertexId, VD)]. This change maintains backwards compatibility and better unifies the VertexRDD methods to match each other.

Author: Brennon York <brennon.york@capitalone.com>

Closes #4733 from brennonyork/SPARK-5922 and squashes the following commits:

e800f08 [Brennon York] fixed merge conflicts
b9274af [Brennon York] fixed merge conflicts
f86375c [Brennon York] fixed minor include line
398ddb4 [Brennon York] fixed merge conflicts
aac1810 [Brennon York] updated to aggregateUsingIndex and added test to ensure that method works properly
2af0b88 [Brennon York] removed deprecation line
753c963 [Brennon York] fixed merge conflicts and set preference to use the diff(other: VertexRDD[VD]) method
2c678c6 [Brennon York] added mima exclude to exclude new public diff method from VertexRDD
93186f3 [Brennon York] added back the original diff method to sustain binary compatibility
f18356e [Brennon York] changed method invocation of 'diff' to match that of 'innerJoin' and 'leftJoin' from VertexRDD[VD] to RDD[(VertexId, VD)]

- MESOS_NATIVE_LIBRARY become deprecated
- Chagned MESOS_NATIVE_LIBRARY to MESOS_NATIVE_JAVA_LIBRARY

Author: Jongyoul Lee <jongyoul@gmail.com>

Closes #4361 from jongyoul/SPARK-3619-1 and squashes the following commits:

f1ea91f [Jongyoul Lee] Merge branch 'SPARK-3619-1' of https://github.com/jongyoul/spark into SPARK-3619-1
a6a00c2 [Jongyoul Lee] [SPARK-3619] Upgrade to Mesos 0.21 to work around MESOS-1688 - Removed 'Known issues' section
2e15a21 [Jongyoul Lee] [SPARK-3619] Upgrade to Mesos 0.21 to work around MESOS-1688 - MESOS_NATIVE_LIBRARY become deprecated - Chagned MESOS_NATIVE_LIBRARY to MESOS_NATIVE_JAVA_LIBRARY
0dace7b [Jongyoul Lee] [SPARK-3619] Upgrade to Mesos 0.21 to work around MESOS-1688 - MESOS_NATIVE_LIBRARY become deprecated - Chagned MESOS_NATIVE_LIBRARY to MESOS_NATIVE_JAVA_LIBRARY

This is a following clean up PR for #5010
This will resolve issues when launching `hive/console` like below:
```
<console>:20: error: object ParquetTestData is not a member of package org.apache.spark.sql.parquet
       import org.apache.spark.sql.parquet.ParquetTestData
```

Author: OopsOutOfMemory <victorshengli@126.com>

Closes #5032 from OopsOutOfMemory/SPARK-6285 and squashes the following commits:

2996aeb [OopsOutOfMemory] remove ParquetTestData

Added tests that maropu [created](https://github.com/maropu/spark/blob/1f64794b2ce33e64f340e383d4e8a60639a7eb4b/graphx/src/test/scala/org/apache/spark/graphx/VertexRDDSuite.scala) for vertices with differing partition counts. Wanted to make sure his work got captured /merged as its not in the master branch and I don't believe there's a PR out already for it.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5023 from brennonyork/SPARK-5790 and squashes the following commits:

83bbd29 [Brennon York] added maropu's tests for vertices with differing partition counts

Updated the configuration docs from the minor items that Reynold had left over from SPARK-1182; specifically I updated the `running-on-mesos` link to point directly to `running-on-mesos#configuration` and upgraded the `yarn`, `mesos`, etc. bullets to `<h5>` tags in hopes that they'll get pushed into the TOC.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5022 from brennonyork/SPARK-6329 and squashes the following commits:

42a10a9 [Brennon York] minor doc fixes

This PR adds a specialized in-memory column type for fixed-precision decimals.

For all other column types, a single integer column type ID is enough to determine which column type to use. However, this doesn't apply to fixed-precision decimal types with different precision and scale parameters. Moreover, according to the previous design, there seems no trivial way to encode precision and scale information into the columnar byte buffer. On the other hand, considering we always know the data type of the column to be built / scanned ahead of time. This PR no longer use column type ID to construct `ColumnBuilder`s and `ColumnAccessor`s, but resorts to the actual column data type. In this way, we can pass precision / scale information along the way.

The column type ID is now not used anymore and can be removed in a future PR.

### Micro benchmark result

The following micro benchmark builds a simple table with 2 million decimals (precision = 10, scale = 0), cache it in memory, then count all the rows. Code (simply paste it into Spark shell):

```scala
import sc._
import sqlContext._
import sqlContext.implicits._
import org.apache.spark.sql.types._
import com.google.common.base.Stopwatch

def benchmark(n: Int)(f: => Long) {
  val stopwatch = new Stopwatch()

  def run() = {
    stopwatch.reset()
    stopwatch.start()
    f
    stopwatch.stop()
    stopwatch.elapsedMillis()
  }

  val records = (0 until n).map(_ => run())

  (0 until n).foreach(i => println(s"Round $i: ${records(i)} ms"))
  println(s"Average: ${records.sum / n.toDouble} ms")
}

// Explicit casting is required because ScalaReflection can't inspect decimal precision
parallelize(1 to 2000000)
  .map(i => Tuple1(Decimal(i, 10, 0)))
  .toDF("dec")
  .select($"dec" cast DecimalType(10, 0))
  .registerTempTable("dec")

sql("CACHE TABLE dec")
val df = table("dec")

// Warm up
df.count()
df.count()

benchmark(5) {
  df.count()
}
```

With `FIXED_DECIMAL` column type:

- Round 0: 75 ms
- Round 1: 97 ms
- Round 2: 75 ms
- Round 3: 70 ms
- Round 4: 72 ms
- Average: 77.8 ms

Without `FIXED_DECIMAL` column type:

- Round 0: 1233 ms
- Round 1: 1170 ms
- Round 2: 1171 ms
- Round 3: 1141 ms
- Round 4: 1141 ms
- Average: 1171.2 ms

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Author: Cheng Lian <lian@databricks.com>

Closes #4938 from liancheng/decimal-column-type and squashes the following commits:

fef5338 [Cheng Lian] Updates fixed decimal column type related test cases
e08ab5b [Cheng Lian] Only resorts to FIXED_DECIMAL when the value can be held in a long
4db713d [Cheng Lian] Adds in-memory column type for fixed-precision decimals

Author: ArcherShao <ArcherShao@users.noreply.github.com>
Author: ArcherShao <shaochuan@huawei.com>

Closes #5007 from ArcherShao/20150313 and squashes the following commits:

ae422ae [ArcherShao] Updated
459efbd [ArcherShao] [SQL]Delete some dupliate code in HiveThriftServer2

use prettyString instead of toString() (which include id of expression) as column name in agg()

Author: Davies Liu <davies@databricks.com>

Closes #5006 from davies/prettystring and squashes the following commits:

cb1fdcf [Davies Liu] use prettyString as column name in agg()

Author: vinodkc <vinod.kc.in@gmail.com>
Author: Vinod K C <vinod.kc@huawei.com>

Closes #5011 from vinodkc/HIVE_console_startupError and squashes the following commits:

b43925f [vinodkc] Changed order of import
b4f5453 [Vinod K C] Fixed HIVE console startup issue

All the contents in this file are not referenced anywhere and should have been removed in #4116 when I tried to get rid of the old Parquet test suites.

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Author: Cheng Lian <lian@databricks.com>

Closes #5010 from liancheng/spark-6285 and squashes the following commits:

06ed057 [Cheng Lian] Removes unused ParquetTestData and duplicated TestGroupWriteSupport

Turns out, per the [convo on the JIRA](https://issues.apache.org/jira/browse/SPARK-4600), `diff` is acting exactly as should. It became a large misconception as I thought it meant set difference, when in fact it does not. To that extent I merely updated the `diff` documentation to, hopefully, better reflect its true intentions moving forward.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5015 from brennonyork/SPARK-4600 and squashes the following commits:

1e1d1e5 [Brennon York] reverted internal diff docs
92288f7 [Brennon York] reverted both the test suite and the diff function back to its origin functionality
f428623 [Brennon York] updated diff documentation to better represent its function
cc16d65 [Brennon York] Merge remote-tracking branch 'upstream/master' into SPARK-4600
66818b9 [Brennon York] added small secondary diff test
99ad412 [Brennon York] Merge remote-tracking branch 'upstream/master' into SPARK-4600
74b8c95 [Brennon York] corrected  method by leveraging bitmask operations to correctly return only the portions of  that are different from the calling VertexRDD
9717120 [Brennon York] updated diff impl to cause fewer objects to be created
710a21c [Brennon York] working diff given test case
aa57f83 [Brennon York] updated to set ShortestPaths to run 'forward' rather than 'backward'

As discussed in the RC3 vote thread, we should mention the change of objective in linear regression in the migration guide. srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #4978 from mengxr/SPARK-6278 and squashes the following commits:

fb3bbe6 [Xiangrui Meng] mention regularization parameter
bfd6cff [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-6278
375fd09 [Xiangrui Meng] address Sean's comments
f87ae71 [Xiangrui Meng] mention step size change

Note: not relevant for Python API since it only has a static train method

Author: Joseph K. Bradley <joseph.kurata.bradley@gmail.com>
Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4969 from jkbradley/SPARK-6252 and squashes the following commits:

a471d90 [Joseph K. Bradley] small edits from review
63eff48 [Joseph K. Bradley] Added getLambda to Scala NaiveBayes

This existed at the very beginning, but became unnecessary after [this commit](https://github.com/apache/spark/commit/37d8f37a8ec110416fba0d51d8ba70370ac380c1#diff-6a9ff7fb74fd490a50462d45db2d5e11L272). I think we should remove it if we don't plan to use it in the future.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #4992 from cloud-fan/small and squashes the following commits:

e857f2e [Wenchen Fan] remove unnecessary ClassTag

For details, please refer to [SPARK-6197](https://issues.apache.org/jira/browse/SPARK-6197)

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #4927 from liyezhang556520/jsonParseError and squashes the following commits:

5cbdc82 [Zhang, Liye] without unnecessary wrap
2b48831 [Zhang, Liye] small changes with sean owen's comments
2973024 [Zhang, Liye] handle json exception when file not finished writing

Also fixed a bunch of minor styling issues.

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Author: Cheng Lian <lian@databricks.com>

Closes #5001 from liancheng/parquet-doc and squashes the following commits:

89ad3db [Cheng Lian] Addresses @rxin's comments
7eb6955 [Cheng Lian] Docs for the new Parquet data source
415eefb [Cheng Lian] Some minor formatting improvements

I've added a timer in the right place to fix this inaccuracy.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #4965 from ilganeli/SPARK-5845 and squashes the following commits:

bfabf88 [Ilya Ganelin] Changed to using a foreach vs. getorelse
3e059b0 [Ilya Ganelin] Switched to using getorelse
b946d08 [Ilya Ganelin] Fixed error with option
9434b50 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-5845
db8647e [Ilya Ganelin] Added update for shuffleWriteTime around spilled file cleanup in ExternalSorter

This fixes a big in the release script and also properly sets things
up so that Zinc launches multiple processes. I had done something
similar in 0c9a8e but it didn't fully work.

Add LassoModel to __all__ in regression.py

LassoModel does not show up in Python docs

This should be merged into branch-1.3 and master.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4970 from jkbradley/SPARK-6253 and squashes the following commits:

c2cb533 [Joseph K. Bradley] Add LassoModel to __all__ in regression.py

This continues the work in #4460 from srowen . The design doc is published on the JIRA page with some minor changes.

Short description of ML attributes: https://github.com/apache/spark/pull/4925/files?diff=unified#diff-95e7f5060429f189460b44a3f8731a35R24

More details can be found in the design doc.

srowen Could you help review this PR? There are many lines but most of them are boilerplate code.

Author: Xiangrui Meng <meng@databricks.com>
Author: Sean Owen <sowen@cloudera.com>

Closes #4925 from mengxr/SPARK-4588-new and squashes the following commits:

71d1bd0 [Xiangrui Meng] add JavaDoc for package ml.attribute
617be40 [Xiangrui Meng] remove final; rename cardinality to numValues
393ffdc [Xiangrui Meng] forgot to include Java attribute group tests
b1aceef [Xiangrui Meng] more tests
e7ab467 [Xiangrui Meng] update ML attribute impl
7c944da [Sean Owen] Add FeatureType hierarchy and categorical cardinality
2a21d6d [Sean Owen] Initial draft of FeatureAttributes class

jira: https://issues.apache.org/jira/browse/SPARK-6268

KMeans has many setters for parameters. It should have matching getters.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #4974 from hhbyyh/get4Kmeans and squashes the following commits:

f44d4dc [Yuhao Yang] add experimental to getRuns
f94a3d7 [Yuhao Yang] add get for KMeans

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5002 from vanzin/mkdist-hotfix and squashes the following commits:

ced65f7 [Marcelo Vanzin] [build] [hotfix] Fix make-distribution.sh for Scala 2.11.

Miss `toDF()` function in docs/sql-programming-guide.md

Author: zzcclp <xm_zzc@sina.com>

Closes #4977 from zzcclp/SPARK-6275 and squashes the following commits:

9a96c7b [zzcclp] Miss toDF()

The link to "Specifying the Hadoop Version" currently points to http://spark.apache.org/docs/latest/building-with-maven.html#specifying-the-hadoop-version.

The correct link is: http://spark.apache.org/docs/latest/building-spark.html#specifying-the-hadoop-version

Author: Theodore Vasiloudis <tvas@sics.se>

Closes #4999 from thvasilo/patch-1 and squashes the following commits:

c34aea8 [Theodore Vasiloudis] Fix dead link in Readme

The issue is discussed in https://issues.apache.org/jira/browse/SPARK-5669. Replacing all JBLAS usage by netlib-java gives us a simpler dependency tree and less license issues to worry about. I didn't touch the test scope in this PR. The user guide is not modified to avoid merge conflicts with branch-1.3. srowen ankurdave pwendell

Author: Xiangrui Meng <meng@databricks.com>

Closes #4699 from mengxr/SPARK-5814 and squashes the following commits:

48635c6 [Xiangrui Meng] move netlib-java version to parent pom
ca21c74 [Xiangrui Meng] remove jblas from ml-guide
5f7767a [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5814
c5c4183 [Xiangrui Meng] merge master
0f20cad [Xiangrui Meng] add mima excludes
e53e9f4 [Xiangrui Meng] remove jblas from mllib runtime
ceaa14d [Xiangrui Meng] replace jblas by netlib-java in graphx
fa7c2ca [Xiangrui Meng] move jblas to test scope

The Thread.interrupt() can not terminate the thread in some cases, so we should not wait for the writerThread of PythonRDD.

This PR also ignore some exception during clean up.

cc JoshRosen mengxr

Author: Davies Liu <davies@databricks.com>

Closes #4987 from davies/fix_take and squashes the following commits:

4488f1a [Davies Liu] fix hang when call take() in JVM on PythonRDD

Author: Volodymyr Lyubinets <vlyubin@gmail.com>

Closes #4988 from vlyubin/columncomp and squashes the following commits:

92d7c8f [Volodymyr Lyubinets] Added equals to Column

Updates to the documentation are as follows:

- Added information on Kafka Direct API and Kafka Python API
- Added joins to the main streaming guide
- Improved details on the fault-tolerance semantics

Generated docs located here
http://people.apache.org/~tdas/spark-1.3.0-temp-docs/streaming-programming-guide.html#fault-tolerance-semantics

More things to add:
- Configuration for Kafka receive rate
- May be add concurrentJobs

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4956 from tdas/streaming-guide-update-1.3 and squashes the following commits:

819408c [Tathagata Das] Minor fixes.
debe484 [Tathagata Das] Added DataFrames and MLlib
380cf8d [Tathagata Das] Fix link
04167a6 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into streaming-guide-update-1.3
0b77486 [Tathagata Das] Updates based on Josh's comments.
86c4c2a [Tathagata Das] Updated streaming guides
82de92a [Tathagata Das] Add Kafka to Python api docs

Added Scala, Java and Python streaming examples showing DataFrame and SQL operations within streaming.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4975 from tdas/streaming-sql-examples and squashes the following commits:

705cba1 [Tathagata Das] Fixed python lint error
75a3fad [Tathagata Das] Fixed python lint error
5fbf789 [Tathagata Das] Removed empty lines at the end
874b943 [Tathagata Das] Added examples streaming + sql examples.

Avoid `UnsupportedOperationException` from JsonRDD.inferSchema on empty RDD.

Not sure if this is supposed to be an error (but a better one), but it seems like this case can come up if the input is down-sampled so much that nothing is sampled.

Now stuff like this:
```
sqlContext.jsonRDD(sc.parallelize(List[String]()))
```
just results in
```
org.apache.spark.sql.DataFrame = []
```

Author: Sean Owen <sowen@cloudera.com>

Closes #4971 from srowen/SPARK-6245 and squashes the following commits:

3699964 [Sean Owen] Set() -> Set.empty
3c619e1 [Sean Owen] Avoid UnsupportedOperationException from JsonRDD.inferSchema on empty RDD

Author: Sandy Ryza <sandy@cloudera.com>

Closes #2490 from sryza/sandy-spark-3642 and squashes the following commits:

aae3340 [Sandy Ryza] SPARK-3642. Document the nuances of broadcast variables