[SPARK-12094][SQL] Prettier tree string for TreeNode

[liancheng]

When examining plans of complex queries with multiple joins, a pain point of mine is that, it's hard to immediately see the sibling node of a specific query plan node. This PR adds tree lines for the tree string of a TreeNode, so that the result can be visually more intuitive.

[liancheng]

For example, the following snippet

val path = "file:///tmp/parquet/unsafe"
(sqlContext range 10 coalesce 1).write mode "overwrite" parquet path
sqlContext.read parquet path registerTempTable "u"
sqlContext.sql(
  """SELECT COUNT(*)
    |FROM (
    |  SELECT x.id FROM u x JOIN u y ON x.id = y.id
    |) a
    |JOIN (
    |  SELECT * FROM u WHERE id < 7
    |) b
    |WHERE a.id = b.id
  """.stripMargin
) explain true

gives the following query plan explanation, which can be (arguably) more intuitive:

...

== Analyzed Logical Plan ==
_c0: bigint
Aggregate [(count(1),mode=Complete,isDistinct=false) AS _c0#23L]
+- Filter (id#0L = id#22L)
   +- Join Inner, None
      :- Subquery a
      :  +- Project [id#0L]
      :     +- Join Inner, Some((id#0L = id#21L))
      :        :- Subquery x
      :        :  +- Subquery u
      :        :     +- Relation[id#0L] ParquetRelation[file:/tmp/parquet/unsafe]
      :        +- Subquery y
      :           +- Subquery u
      :              +- Relation[id#21L] ParquetRelation[file:/tmp/parquet/unsafe]
      +- Subquery b
         +- Project [id#22L]
            +- Filter (id#22L < cast(7 as bigint))
               +- Subquery u
                  +- Relation[id#22L] ParquetRelation[file:/tmp/parquet/unsafe]

...

The reason why ":" is chosen instead of "|" is that, "|" doesn't play well with the following pattern (which is used extensively in Spark SQL), because "|" is the default margin character in Scala.

"""...
  |plan.treeString
  |...
""".stripMargin

[cloud-fan]

this may be arguably more readable:

lastChildren.init.map { isLast =>
  if (isLast) "   " else ":  "
}.mkString + (if (lastChildren.last) "+- " else ":- ")

[cloud-fan]

This is pretty cool, thanks! LGTM

[liancheng]

@cloud-fan Thanks! Comments addressed.

[SparkQA]

Test build #47053 has finished for PR 10099 at commit 90b46d9.

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

[SparkQA]

Test build #47062 has finished for PR 10099 at commit 10eb09e.

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

[SparkQA]

Test build #47063 has finished for PR 10099 at commit 31a1c22.

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

[yhuai]

Merging to master and branch 1.6.