[SPARK-12536] [SQL] Added "[]" in Explain Outputs For Empty LocalRelation and LocalTableScan

[gatorsmile]

The filter Filter (False) generates an empty LocalRelation in SimplifyFilters. In the current Explain, the optimized and physical plans look confusing.

For example,

val df = Seq(1 -> "a").toDF("a", "b")
df.where("1 = 0").explain(true)

== Analyzed Logical Plan ==
a: int, b: string
Filter (1 = 0)
+- Project [_1#0 AS a#2,_2#1 AS b#3]
   +- LocalRelation [_1#0,_2#1], [[1,a]]

== Optimized Logical Plan ==
LocalRelation [a#2,b#3] 

== Physical Plan ==
LocalTableScan [a#2,b#3] 

After the fix, the Optimized Logical Plan and Physical Plan are changed to

== Optimized Logical Plan ==
LocalRelation [a#2,b#3] []

== Physical Plan ==
LocalTableScan [a#2,b#3] [] 

The original PR is: #10494

[gatorsmile]

@cloud-fan @marmbrus Hopefully, my understanding is right. Feel free to let me know if the fix needs any change. Thank you!

[SparkQA]

Test build #48946 has finished for PR 10646 at commit 3ac9735.

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

[cloud-fan]

I think a better way is to change this if to if seq.nonEmpty && seq.toSet.subsetOf(children.toSet)

[gatorsmile]

Thank you! Will do!

[SparkQA]

Test build #49091 has finished for PR 10646 at commit 4ae1ce7.

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

[SparkQA]

Test build #49092 has finished for PR 10646 at commit 18091f7.

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

[gatorsmile]

retest this please.

[SparkQA]

Test build #49152 has finished for PR 10646 at commit 18091f7.

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

[cloud-fan]

these import changes are irrelevant？

[gatorsmile]

Yeah, I just saw these useless imports in this file. Do you want me to keep them unchanged?

[cloud-fan]

oh, it's fine to remove useless imports.

[cloud-fan]

LGTM

[gatorsmile]

Thank you for your help! Sorry for wasting you a lot of time on it! @cloud-fan

[rxin]

@gatorsmile I'm actually not sure whether the new format is more clear. Do you mind closing this? Thanks.

[gatorsmile]

Sure, let me close it.