[SPARK-16543][SQL] Rename the columns of SHOW PARTITION/COLUMNS commands

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR changes the name of columns returned by SHOW PARTITION and SHOW COLUMNS commands. Currently, both commands uses result as a column name.

Comparison: Column Name

Command	Spark(Before)	Spark(After)	Hive
SHOW PARTITIONS	result	partition	partition
SHOW COLUMNS	result	col_name	field

Note that Spark/Hive uses col_name in DESC TABLES. So, this PR chooses col_name for consistency among Spark commands.

Before

scala> sql("show partitions p").show()
+------+
|result|
+------+
|   b=2|
+------+

scala> sql("show columns in p").show()
+------+
|result|
+------+
|     a|
|     b|
+------+

After

scala> sql("show partitions p").show
+---------+
|partition|
+---------+
|      b=2|
+---------+

scala> sql("show columns in p").show
+--------+
|col_name|
+--------+
|       a|
|       b|
+--------+

How was this patch tested?

Manual.

[SparkQA]

Test build #62307 has finished for PR 14199 at commit 1f6ba91.

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

[hvanhovell]

LGTM - merging to master. Thanks.

[dongjoon-hyun]

Thank you, @hvanhovell