[SPARK-16331] [SQL] Reduce code generation time

[inouehrs]

What changes were proposed in this pull request?

During the code generation, a LocalRelation often has a huge Vector object as data. In the simple example below, a LocalRelation has a Vector with 1000000 elements of UnsafeRow.

val numRows = 1000000
val ds = (1 to numRows).toDS().persist()
benchmark.addCase("filter+reduce") { iter =>
  ds.filter(a => (a & 1) == 0).reduce(_ + _)
}

At TreeNode.transformChildren, all elements of the vector is unnecessarily iterated to check whether any children exist in the vector since Vector is Traversable. This part significantly increases code generation time.

This patch avoids this overhead by checking the number of children before iterating all elements; LocalRelation does not have children since it extends LeafNode.

The performance of the above example

without this patch
Java HotSpot(TM) 64-Bit Server VM 1.8.0_91-b14 on Mac OS X 10.11.5
Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
compilationTime:                         Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
filter+reduce                                 4426 / 4533          0.2        4426.0       1.0X

with this patch
compilationTime:                         Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
filter+reduce                                 3117 / 3391          0.3        3116.6       1.0X

How was this patch tested?

using existing unit tests

[marmbrus]

ok to test

[rxin]

a small style nit:

} else {
  this
}

[rxin]

LGTM other than the small nit.

The actual diff is very small: https://github.com/apache/spark/pull/14000/files?w=1

[SparkQA]

Test build #61567 has finished for PR 14000 at commit 153e170.

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

[inouehrs]

I fixed the coding style issue.

[SparkQA]

Test build #61586 has finished for PR 14000 at commit 639090d.

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

[viirya]

LGTM. good catch!

[rxin]

Merging in master. Thanks.