[SPARK-2176][SQL] Extra unnecessary exchange operator in the result of an explain command

```
hql("explain select * from src group by key").collect().foreach(println)

[ExplainCommand [plan#27:0]]
[ Aggregate false, [key#25], [key#25,value#26]]
[  Exchange (HashPartitioning [key#25:0], 200)]
[   Exchange (HashPartitioning [key#25:0], 200)]
[    Aggregate true, [key#25], [key#25]]
[     HiveTableScan [key#25,value#26], (MetastoreRelation default, src, None), None]
```

There are two exchange operators.

However, if we do not use explain...
```
hql("select * from src group by key")

res4: org.apache.spark.sql.SchemaRDD =
SchemaRDD[8] at RDD at SchemaRDD.scala:100
== Query Plan ==
Aggregate false, [key#8], [key#8,value#9]
 Exchange (HashPartitioning [key#8:0], 200)
  Aggregate true, [key#8], [key#8]
   HiveTableScan [key#8,value#9], (MetastoreRelation default, src, None), None
```
The plan is fine.

The cause of this bug is explained below.

When we create an `execution.ExplainCommand`, we use the `executedPlan` as the child of this `ExplainCommand`. But, this `executedPlan` is prepared for execution again when we generate the `executedPlan` for the `ExplainCommand`. Basically, `prepareForExecution` is called twice on a physical plan. Because after `prepareForExecution` we have already bounded those references (in `BoundReference`s), `AddExchange` cannot figure out we are using the same partitioning (we use `AttributeReference`s to create an `ExchangeOperator` and then those references will be changed to `BoundReference`s after `prepareForExecution` is called). So, an extra `ExchangeOperator` is inserted.

I think in `CommandStrategy`, we should just use the `sparkPlan` (`sparkPlan` is the input of `prepareForExecution`) to initialize the `ExplainCommand` instead of using `executedPlan`.

The link to JIRA: https://issues.apache.org/jira/browse/SPARK-2176

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

Closes #1116 from yhuai/SPARK-2176 and squashes the following commits:

197c19c [Yin Huai] Use sparkPlan to initialize a Physical Explain Command instead of using executedPlan.
[SPARK-2162] Double check in doGetLocal to avoid read on removed block.

other wise, it will either read in vain in memory level case, or throw exception in disk level case when it believe the block is there while actually it had been removed.

Author: Raymond Liu <raymond.liu@intel.com>

Closes #1103 from colorant/bm and squashes the following commits:

daac114 [Raymond Liu] Address comments
d1ea287 [Raymond Liu] Double check in doGetLocal to avoid read on removed block.
Updated the comment for SPARK-2162.

A follow up on #1103

@andrewor14

Author: Reynold Xin <rxin@apache.org>

Closes #1117 from rxin/SPARK-2162 and squashes the following commits:

a4231de [Reynold Xin] Updated the comment for SPARK-2162.
[SPARK-1466] Raise exception if pyspark Gateway process doesn't start.

If the gateway process fails to start correctly (e.g., because JAVA_HOME isn't set correctly, there's no Spark jar, etc.), right now pyspark fails because of a very difficult-to-understand error, where we try to parse stdout to get the port where Spark started and there's nothing there. This commit properly catches the error and throws an exception that includes the stderr output for much easier debugging.

Thanks to @shivaram and @stogers for helping to fix this issue!

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #383 from kayousterhout/pyspark and squashes the following commits:

36dd54b [Kay Ousterhout] [SPARK-1466] Raise exception if Gateway process doesn't start.
SPARK-2158 Clean up core/stdout file from FileAppenderSuite

@tdas

Author: Mark Hamstra <markhamstra@gmail.com>

Closes #1100 from markhamstra/SPARK-2158 and squashes the following commits:

ae8e069 [Mark Hamstra] Response to TD's review
2f1e201 [Mark Hamstra] Cleanup 'stdout' file within FileAppenderSuite
Remove unicode operator from RDD.scala

Some IDEs don’t support unicode characters in source code. Check if this breaks binary compatibility.

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1119 from dorx/unicode and squashes the following commits:

05618c3 [Doris Xin] Remove unicode operator from RDD.scala
[SPARK-2184][SQL] AddExchange isn't idempotent

...redPartitioning.

Author: Michael Armbrust <michael@databricks.com>

Closes #1122 from marmbrus/fixAddExchange and squashes the following commits:

3417537 [Michael Armbrust] Don't bind partitioning expressions as that breaks comparison with requiredPartitioning.
Squishing a typo bug before it causes real harm

in updateNumRows method in RowMatrix

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1125 from dorx/updateNumRows and squashes the following commits:

8564aef [Doris Xin] Squishing a typo bug before it causes real harm
[SPARK-2187] Explain should not run the optimizer twice.

@yhuai @marmbrus @concretevitamin

Author: Reynold Xin <rxin@apache.org>

Closes #1123 from rxin/explain and squashes the following commits:

def83b0 [Reynold Xin] Update unit tests for explain.
a9d3ba8 [Reynold Xin] [SPARK-2187] Explain should not run the optimizer twice.
Minor fix

The value "env" is never used in SparkContext.scala.
Add detailed comment for method setDelaySeconds in MetadataCleaner.scala instead of the unsure one.

Author: WangTao <barneystinson@aliyun.com>

Closes #1105 from WangTaoTheTonic/master and squashes the following commits:

688358e [WangTao] Minor fix
[SPARK-2051]In yarn.ClientBase spark.yarn.dist.* do not work

Author: witgo <witgo@qq.com>

Closes #969 from witgo/yarn_ClientBase and squashes the following commits:

8117765 [witgo] review commit
3bdbc52 [witgo] Merge branch 'master' of https://github.com/apache/spark into yarn_ClientBase
5261b6c [witgo] fix sys.props.get("SPARK_YARN_DIST_FILES")
e3c1107 [witgo] update docs
b6a9aa1 [witgo] merge master
c8b4554 [witgo] review commit
2f48789 [witgo] Merge branch 'master' of https://github.com/apache/spark into yarn_ClientBase
8d7b82f [witgo] Merge branch 'master' of https://github.com/apache/spark into yarn_ClientBase
1048549 [witgo] remove Utils.resolveURIs
871f1db [witgo] add spark.yarn.dist.* documentation
41bce59 [witgo] review commit
35d6fa0 [witgo] move to ClientArguments
55d72fc [witgo] Merge branch 'master' of https://github.com/apache/spark into yarn_ClientBase
9cdff16 [witgo] review commit
8bc2f4b [witgo] review commit
20e667c [witgo] Merge branch 'master' into yarn_ClientBase
0961151 [witgo] merge master
ce609fc [witgo] Merge branch 'master' into yarn_ClientBase
8362489 [witgo] yarn.ClientBase spark.yarn.dist.* do not work
[SPARK-2191][SQL] Make sure InsertIntoHiveTable doesn't execute more than once.

Author: Michael Armbrust <michael@databricks.com>

Closes #1129 from marmbrus/doubleCreateAs and squashes the following commits:

9c6d9e4 [Michael Armbrust] Fix typo.
5128fe2 [Michael Armbrust] Make sure InsertIntoHiveTable doesn't execute each time you ask for its result.
[SPARK-2151] Recognize memory format for spark-submit

int format expected for input memory parameter when spark-submit is invoked in standalone cluster mode. Make it consistent with rest of Spark.

Author: nravi <nravi@c1704.halxg.cloudera.com>

Closes #1095 from nishkamravi2/master and squashes the following commits:

2b630f9 [nravi] Accept memory input as "30g", "512M" instead of an int value, to be consistent with rest of Spark
3bf8fad [nravi] Merge branch 'master' of https://github.com/apache/spark
5423a03 [nravi] Merge branch 'master' of https://github.com/apache/spark
eb663ca [nravi] Merge branch 'master' of https://github.com/apache/spark
df2aeb1 [nravi] Improved fix for ConcurrentModificationIssue (Spark-1097, Hadoop-10456)
6b840f0 [nravi] Undo the fix for SPARK-1758 (the problem is fixed)
5108700 [nravi] Fix in Spark for the Concurrent thread modification issue (SPARK-1097, HADOOP-10456)
681b36f [nravi] Fix for SPARK-1758: failing test org.apache.spark.JavaAPISuite.wholeTextFiles
A few minor Spark SQL Scaladoc fixes.

Author: Reynold Xin <rxin@apache.org>

Closes #1139 from rxin/sparksqldoc and squashes the following commits:

c3049d8 [Reynold Xin] Fixed line length.
66dc72c [Reynold Xin] A few minor Spark SQL Scaladoc fixes.
HOTFIX: SPARK-2208 local metrics tests can fail on fast machines

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1141 from pwendell/hotfix and squashes the following commits:

83e4c79 [Patrick Wendell] HOTFIX: SPARK-2208 local metrics tests can fail on fast machines
More minor scaladoc cleanup for Spark SQL.

Author: Reynold Xin <rxin@apache.org>

Closes #1142 from rxin/sqlclean and squashes the following commits:

67a789e [Reynold Xin] More minor scaladoc cleanup for Spark SQL.
[SQL] Improve Speed of InsertIntoHiveTable

Author: Michael Armbrust <michael@databricks.com>

Closes #1130 from marmbrus/noFunctional and squashes the following commits:

ccdb68c [Michael Armbrust] Remove functional programming and Array allocations from fast path in InsertIntoHiveTable.
[SPARK-2177][SQL] describe table result contains only one column

```
scala> hql("describe src").collect().foreach(println)

[key                 	string              	None                ]
[value               	string              	None                ]
```

The result should contain 3 columns instead of one. This screws up JDBC or even the downstream consumer of the Scala/Java/Python APIs.

I am providing a workaround. We handle a subset of describe commands in Spark SQL, which are defined by ...
```
DESCRIBE [EXTENDED] [db_name.]table_name
```
All other cases are treated as Hive native commands.

Also, if we upgrade Hive to 0.13, we need to check the results of context.sessionState.isHiveServerQuery() to determine how to split the result. This method is introduced by https://issues.apache.org/jira/browse/HIVE-4545. We may want to set Hive to use JsonMetaDataFormatter for the output of a DDL statement (`set hive.ddl.output.format=json` introduced by https://issues.apache.org/jira/browse/HIVE-2822).

The link to JIRA: https://issues.apache.org/jira/browse/SPARK-2177

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

Closes #1118 from yhuai/SPARK-2177 and squashes the following commits:

fd2534c [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
b9b9aa5 [Yin Huai] rxin's comments.
e7c4e72 [Yin Huai] Fix unit test.
656b068 [Yin Huai] 100 characters.
6387217 [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
8003cf3 [Yin Huai] Generate strings with the format like Hive for unit tests.
9787fff [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
440c5af [Yin Huai] rxin's comments.
f1a417e [Yin Huai] Update doc.
83adb2f [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
366f891 [Yin Huai] Add describe command.
74bd1d4 [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
342fdf7 [Yin Huai] Split to up to 3 parts.
725e88c [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
bb8bbef [Yin Huai] Split every string in the result of a describe command.
SPARK-1293 [SQL] Parquet support for nested types

It should be possible to import and export data stored in Parquet's columnar format that contains nested types. For example:
```java
message AddressBook {
   required binary owner;
   optional group ownerPhoneNumbers {
      repeated binary array;
   }
   optional group contacts {
      repeated group array {
         required binary name;
         optional binary phoneNumber;
      }
   }
   optional group nameToApartmentNumber {
      repeated group map {
         required binary key;
         required int32 value;
      }
   }
}
```
The example could model a type (AddressBook) that contains records made of strings (owner), lists (ownerPhoneNumbers) and a table of contacts (e.g., a list of pairs or a map that can contain null values but keys must not be null). The list of tasks are as follows:

<h6>Implement support for converting nested Parquet types to Spark/Catalyst types:</h6>
- [x] Structs
- [x] Lists
- [x] Maps (note: currently keys need to be Strings)

<h6>Implement import (via ``parquetFile``) of nested Parquet types (first version in this PR)</h6>
- [x] Initial version

<h6>Implement export (via ``saveAsParquetFile``)</h6>
- [x] Initial version

<h6>Test support for AvroParquet, etc.</h6>
- [x] Initial testing of import of avro-generated Parquet data (simple + nested)

Example:
```scala
val data = TestSQLContext
  .parquetFile("input.dir")
  .toSchemaRDD
data.registerAsTable("data")
sql("SELECT owner, contacts[1].name, nameToApartmentNumber['John'] FROM data").collect()
```

Author: Andre Schumacher <andre.schumacher@iki.fi>
Author: Michael Armbrust <michael@databricks.com>

Closes #360 from AndreSchumacher/nested_parquet and squashes the following commits:

30708c8 [Andre Schumacher] Taking out AvroParquet test for now to remove Avro dependency
95c1367 [Andre Schumacher] Changes to ParquetRelation and its metadata
7eceb67 [Andre Schumacher] Review feedback
94eea3a [Andre Schumacher] Scalastyle
403061f [Andre Schumacher] Fixing some issues with tests and schema metadata
b8a8b9a [Andre Schumacher] More fixes to short and byte conversion
63d1b57 [Andre Schumacher] Cleaning up and Scalastyle
88e6bdb [Andre Schumacher] Attempting to fix loss of schema
37e0a0a [Andre Schumacher] Cleaning up
14c3fd8 [Andre Schumacher] Attempting to fix Spark-Parquet schema conversion
3e1456c [Michael Armbrust] WIP: Directly serialize catalyst attributes.
f7aeba3 [Michael Armbrust] [SPARK-1982] Support for ByteType and ShortType.
3104886 [Michael Armbrust] Nested Rows should be Rows, not Seqs.
3c6b25f [Andre Schumacher] Trying to reduce no-op changes wrt master
31465d6 [Andre Schumacher] Scalastyle: fixing commented out bottom
de02538 [Andre Schumacher] Cleaning up ParquetTestData
2f5a805 [Andre Schumacher] Removing stripMargin from test schemas
191bc0d [Andre Schumacher] Changing to Seq for ArrayType, refactoring SQLParser for nested field extension
cbb5793 [Andre Schumacher] Code review feedback
32229c7 [Andre Schumacher] Removing Row nested values and placing by generic types
0ae9376 [Andre Schumacher] Doc strings and simplifying ParquetConverter.scala
a6b4f05 [Andre Schumacher] Cleaning up ArrayConverter, moving classTag to NativeType, adding NativeRow
431f00f [Andre Schumacher] Fixing problems introduced during rebase
c52ff2c [Andre Schumacher] Adding native-array converter
619c397 [Andre Schumacher] Completing Map testcase
79d81d5 [Andre Schumacher] Replacing field names for array and map in WriteSupport
f466ff0 [Andre Schumacher] Added ParquetAvro tests and revised Array conversion
adc1258 [Andre Schumacher] Optimizing imports
e99cc51 [Andre Schumacher] Fixing nested WriteSupport and adding tests
1dc5ac9 [Andre Schumacher] First version of WriteSupport for nested types
d1911dc [Andre Schumacher] Simplifying ArrayType conversion
f777b4b [Andre Schumacher] Scalastyle
824500c [Andre Schumacher] Adding attribute resolution for MapType
b539fde [Andre Schumacher] First commit for MapType
a594aed [Andre Schumacher] Scalastyle
4e25fcb [Andre Schumacher] Adding resolution of complex ArrayTypes
f8f8911 [Andre Schumacher] For primitive rows fall back to more efficient converter, code reorg
6dbc9b7 [Andre Schumacher] Fixing some problems intruduced during rebase
b7fcc35 [Andre Schumacher] Documenting conversions, bugfix, wrappers of Rows
ee70125 [Andre Schumacher] fixing one problem with arrayconverter
98219cf [Andre Schumacher] added struct converter
5d80461 [Andre Schumacher] fixing one problem with nested structs and breaking up files
1b1b3d6 [Andre Schumacher] Fixing one problem with nested arrays
ddb40d2 [Andre Schumacher] Extending tests for nested Parquet data
745a42b [Andre Schumacher] Completing testcase for nested data (Addressbook(
6125c75 [Andre Schumacher] First working nested Parquet record input
4d4892a [Andre Schumacher] First commit nested Parquet read converters
aa688fe [Andre Schumacher] Adding conversion of nested Parquet schemas
[SPARK-2210] cast to boolean on boolean value gets turned into NOT((boolean_condition) = 0)

```
explain select cast(cast(key=0 as boolean) as boolean) aaa from src
```
should be
```
[Physical execution plan:]
[Project [(key#10:0 = 0) AS aaa#7]]
[ HiveTableScan [key#10], (MetastoreRelation default, src, None), None]
```

However, it is currently
```
[Physical execution plan:]
[Project [NOT((key#10=0) = 0) AS aaa#7]]
[ HiveTableScan [key#10], (MetastoreRelation default, src, None), None]
```

Author: Reynold Xin <rxin@apache.org>

Closes #1144 from rxin/booleancast and squashes the following commits:

c4e543d [Reynold Xin] [SPARK-2210] boolean cast on boolean value should be removed.