Add/increase severity of warning in documentation of groupBy()

groupBy()/groupByKey() is notorious for being a very convenient API that can lead to poor performance when used incorrectly.

This PR just makes it clear that users should be cautious not to rely on this API when they really want a different (more performant) one, such as reduceByKey().

(Note that one source of confusion is the name; this groupBy() is not the same as a SQL GROUP-BY, which is used for aggregation and is more similar in nature to Spark's reduceByKey().)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1380 from aarondav/warning and squashes the following commits:

f60da39 [Aaron Davidson] Give better advice
d0afb68 [Aaron Davidson] Add/increase severity of warning in documentation of groupBy()
(cherry picked from commit a2aa7bebae31e1e7ec23d31aaa436283743b283b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2485][SQL] Lock usage of hive client.

Author: Michael Armbrust <michael@databricks.com>

Closes #1412 from marmbrus/lockHiveClient and squashes the following commits:

4bc9d5a [Michael Armbrust] protected[hive]
22e9177 [Michael Armbrust] Add comments.
7aa8554 [Michael Armbrust] Don't lock on hive's object.
a6edc5f [Michael Armbrust] Lock usage of hive client.

(cherry picked from commit c7c7ac83392b10abb011e6aead1bf92e7c73695e)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
[SPARK-2483][SQL] Fix parsing of repeated, nested data access.

Author: Michael Armbrust <michael@databricks.com>

Closes #1411 from marmbrus/nestedRepeated and squashes the following commits:

044fa09 [Michael Armbrust] Fix parsing of repeated, nested data access.

(cherry picked from commit 0f98ef1a2c9ecf328f6c5918808fa5ca486e8afd)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SQL] Whitelist more Hive tests.

Author: Michael Armbrust <michael@databricks.com>

Closes #1396 from marmbrus/moreTests and squashes the following commits:

6660b60 [Michael Armbrust] Blacklist a test that requires DFS command.
8b6001c [Michael Armbrust] Add golden files.
ccd8f97 [Michael Armbrust] Whitelist more tests.

(cherry picked from commit bcd0c30c7eea4c50301cb732c733fdf4d4142060)
Signed-off-by: Michael Armbrust <michael@databricks.com>
SPARK-2407: Added internal implementation of SQL SUBSTR()

This replaces the Hive UDF for SUBSTR(ING) with an implementation in Catalyst
and adds tests to verify correct operation.

Author: William Benton <willb@redhat.com>

Closes #1359 from willb/internalSqlSubstring and squashes the following commits:

ccedc47 [William Benton] Fixed too-long line.
a30a037 [William Benton] replace view bounds with implicit parameters
ec35c80 [William Benton] Adds fixes from review:
4f3bfdb [William Benton] Added internal implementation of SQL SUBSTR()

(cherry picked from commit 61de65bc69f9a5fc396b76713193c6415436d452)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SQL] Attribute equality comparisons should be done by exprId.

Author: Michael Armbrust <michael@databricks.com>

Closes #1414 from marmbrus/exprIdResolution and squashes the following commits:

97b47bc [Michael Armbrust] Attribute equality comparisons should be done by exprId.

(cherry picked from commit 502f90782ad474e2630ed5be4d3c4be7dab09c34)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-2498] [SQL] Synchronize on a lock when using scala reflection inside data type objects.

JIRA ticket: https://issues.apache.org/jira/browse/SPARK-2498

Author: Zongheng Yang <zongheng.y@gmail.com>

Closes #1423 from concretevitamin/scala-ref-catalyst and squashes the following commits:

325a149 [Zongheng Yang] Synchronize on a lock when initializing data type objects in Catalyst.

(cherry picked from commit c2048a5165b270f5baf2003fdfef7bc6c5875715)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-2314][SQL] Override collect and take in JavaSchemaRDD, forwarding to SchemaRDD implementations.

Author: Aaron Staple <aaron.staple@gmail.com>

Closes #1421 from staple/SPARK-2314 and squashes the following commits:

73e04dc [Aaron Staple] [SPARK-2314] Override collect and take in JavaSchemaRDD, forwarding to SchemaRDD implementations.

(cherry picked from commit 90ca532a0fd95dc85cff8c5722d371e8368b2687)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2509][SQL] Add optimization for Substring.

`Substring` including `null` literal cases could be added to `NullPropagation`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1428 from ueshin/issues/SPARK-2509 and squashes the following commits:

d9eb85f [Takuya UESHIN] Add Substring cases to NullPropagation.

(cherry picked from commit 9b38b7c71352bb5e6d359515111ad9ca33299127)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2504][SQL] Fix nullability of Substring expression.

This is a follow-up of #1359 with nullability narrowing.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1426 from ueshin/issues/SPARK-2504 and squashes the following commits:

5157832 [Takuya UESHIN] Remove unnecessary white spaces.
80958ac [Takuya UESHIN] Fix nullability of Substring expression.

(cherry picked from commit 632fb3d9a9ebb3d2218385403145d5b89c41c025)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2525][SQL] Remove as many compilation warning messages as possible in Spark SQL

JIRA: https://issues.apache.org/jira/browse/SPARK-2525.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1444 from yhuai/SPARK-2517 and squashes the following commits:

edbac3f [Yin Huai] Removed some compiler type erasure warnings.

(cherry picked from commit df95d82da7c76c074fd4064f7c870d55d99e0d8e)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2518][SQL] Fix foldability of Substring expression.

This is a follow-up of #1428.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1432 from ueshin/issues/SPARK-2518 and squashes the following commits:

37d1ace [Takuya UESHIN] Fix foldability of Substring expression.

(cherry picked from commit cc965eea510397642830acb21f61127b68c098d6)
Signed-off-by: Reynold Xin <rxin@apache.org>
SPARK-1097: Do not introduce deadlock while fixing concurrency bug

We recently added this lock on 'conf' in order to prevent concurrent creation. However, it turns out that this can introduce a deadlock because Hadoop also synchronizes on the Configuration objects when creating new Configurations (and they do so via a static REGISTRY which contains all created Configurations).

This fix forces all Spark initialization of Configuration objects to occur serially by using a static lock that we control, and thus also prevents introducing the deadlock.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1409 from aarondav/1054 and squashes the following commits:

7d1b769 [Aaron Davidson] SPARK-1097: Do not introduce deadlock while fixing concurrency bug
(cherry picked from commit 8867cd0bc2961fefed84901b8b14e9676ae6ab18)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2154] Schedule next Driver when one completes (standalone mode)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1405 from aarondav/2154 and squashes the following commits:

24e9ef9 [Aaron Davidson] [SPARK-2154] Schedule next Driver when one completes (standalone mode)
(cherry picked from commit 9c249743eaabe5fc8d961c7aa581cc0197f6e950)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2412] CoalescedRDD throws exception with certain pref locs

If the first pass of CoalescedRDD does not find the target number of locations AND the second pass finds new locations, an exception is thrown, as "groupHash.get(nxt_replica).get" is not valid.

The fix is just to add an ArrayBuffer to groupHash for that replica if it didn't already exist.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1337 from aarondav/2412 and squashes the following commits:

f587b5d [Aaron Davidson] getOrElseUpdate
3ad8a3c [Aaron Davidson] [SPARK-2412] CoalescedRDD throws exception with certain pref locs
(cherry picked from commit 7c23c0dc3ed721c95690fc49f435d9de6952523c)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2534] Avoid pulling in the entire RDD in various operators (branch-1.0 backport)

This backports #1450 into branch-1.0.

Author: Reynold Xin <rxin@apache.org>

Closes #1469 from rxin/closure-1.0 and squashes the following commits:

b474a92 [Reynold Xin] [SPARK-2534] Avoid pulling in the entire RDD in various operators
[SPARK-2570] [SQL] Fix the bug of ClassCastException

Exception thrown when running the example of HiveFromSpark.
Exception in thread "main" java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:106)
	at org.apache.spark.sql.catalyst.expressions.GenericRow.getInt(Row.scala:145)
	at org.apache.spark.examples.sql.hive.HiveFromSpark$.main(HiveFromSpark.scala:45)
	at org.apache.spark.examples.sql.hive.HiveFromSpark.main(HiveFromSpark.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.spark.deploy.SparkSubmit$.launch(SparkSubmit.scala:303)
	at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:55)
	at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

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

Closes #1475 from chenghao-intel/hive_from_spark and squashes the following commits:

d4c0500 [Cheng Hao] Fix the bug of ClassCastException

(cherry picked from commit 29809a6d58bfe3700350ce1988ff7083881c4382)
Signed-off-by: Reynold Xin <rxin@apache.org>
Added t2 instance types

New t2 instance types require HVM amis, bailout assumption of pvm
causes failures when using t2 instance types.

Author: Basit Mustafa <basitmustafa@computes-things-for-basit.local>

Closes #1446 from 24601/master and squashes the following commits:

01fe128 [Basit Mustafa] Makin' it pretty
392a95e [Basit Mustafa] Added t2 instance types

Conflicts:

	ec2/spark_ec2.py
[SPARK-2540] [SQL] Add HiveDecimal & HiveVarchar support in unwrapping data

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

Closes #1436 from chenghao-intel/unwrapdata and squashes the following commits:

34cc21a [Cheng Hao] update the table scan accodringly since the unwrapData function changed
afc39da [Cheng Hao] Polish the code
39d6475 [Cheng Hao] Add HiveDecimal & HiveVarchar support in unwrap data

(cherry picked from commit 7f1720813793e155743b58eae5228298e894b90d)
Signed-off-by: Michael Armbrust <michael@databricks.com>
Typo fix to the programming guide in the docs

Typo fix to the programming guide in the docs. Changed the word "distibuted" to "distributed".

Author: Cesar Arevalo <cesar@zephyrhealthinc.com>

Closes #1495 from cesararevalo/master and squashes the following commits:

0c2e3a7 [Cesar Arevalo] Typo fix to the programming guide in the docs
(cherry picked from commit 0d01e85f42f3c997df7fee942b05b509968bac4b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>