[SPARK-14415][SQL] All functions should show usages by command DESC FUNCTION

[dongjoon-hyun]

What changes were proposed in this pull request?

Currently, many functions do now show usages like the followings.

scala> sql("desc function extended `sin`").collect().foreach(println)
[Function: sin]
[Class: org.apache.spark.sql.catalyst.expressions.Sin]
[Usage: To be added.]
[Extended Usage:
To be added.]

This PR adds descriptions for functions and adds a testcase prevent adding function without usage.

scala>  sql("desc function extended `sin`").collect().foreach(println);
[Function: sin]
[Class: org.apache.spark.sql.catalyst.expressions.Sin]
[Usage: sin(x) - Returns the sine of x.]
[Extended Usage:
> SELECT sin(0);
 0.0]

The only exceptions are cube, grouping, grouping_id, rollup, window.

How was this patch tested?

Pass the Jenkins tests (including new testcases.)

[SparkQA]

Test build #55030 has finished for PR 12185 at commit 1bc40e1.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55039 has finished for PR 12185 at commit 26a1865.

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

[SparkQA]

Test build #55084 has finished for PR 12185 at commit 2d7cee8.

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

[SparkQA]

Test build #55176 has finished for PR 12185 at commit 171389e.

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

[SparkQA]

Test build #55283 has finished for PR 12185 at commit b1d5cd1.

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

[SparkQA]

Test build #55304 has finished for PR 12185 at commit e41326b.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55313 has finished for PR 12185 at commit 68c90a0.

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

[dongjoon-hyun]

Hi, @rxin .
Could you give me some directional advice to improve this PR?

[rxin]

This is super useful.

@yhuai can you review this?

[dongjoon-hyun]

Thank you so much! @rxin :)

[yhuai]

What do you mean by (array, value)?

[yhuai]

Maybe Returns the average of values in a group?

[dongjoon-hyun]

Oh, @yhuai . Thank you for deep and detail review!
I'll fix all of them and improve as possible as I can.

[yhuai]

For those aggregate functions, I feel it is better to mention that they are calculated from values of a group instead of just saying from a set of values?

[dongjoon-hyun]

Sure. You mean
'Returns the XXX of a set of numbers' -> 'Returns the XXX calculated from values of a group'
Right?

[dongjoon-hyun]

Since I just borrowed from the Hive cli, it seems to be weird. I think we can improve them in this PR as much as possible.

[yhuai]

Yea. I feel that calculated from values of a group is a little bit better.

[dongjoon-hyun]

Thank you. I agree. I will apply them all related descriptions.

[SparkQA]

Test build #55454 has finished for PR 12185 at commit 03d3395.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55455 has finished for PR 12185 at commit 080891f.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55467 has finished for PR 12185 at commit d88121c.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55468 has finished for PR 12185 at commit ff20b07.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55477 has finished for PR 12185 at commit d0f5ed6.

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

[yhuai]

Thanks. Merging to master.

[dongjoon-hyun]

Thank you, @yhuai and @rxin .