c233ab3d8d
In Scala, `map` and `flatMap` of `Iterable` will copy the contents of `Iterable` to a new `Seq`. Such as,
```Scala
val iterable = Seq(1, 2, 3).map(v => {
println(v)
v
})
println("Iterable map done")
val iterator = Seq(1, 2, 3).iterator.map(v => {
println(v)
v
})
println("Iterator map done")
```
outputed
```
1
2
3
Iterable map done
Iterator map done
```
So we should use 'iterator' to reduce memory consumed by join.
Found by Johannes Simon in http://mail-archives.apache.org/mod_mbox/spark-user/201412.mbox/%3C5BE70814-9D03-4F61-AE2C-0D63F2DE4446%40mail.de%3E
Author: zsxwing <zsxwing@gmail.com>
Closes #3671 from zsxwing/SPARK-4824 and squashes the following commits:
48ee7b9 [zsxwing] Remove the explicit types
95d59d6 [zsxwing] Add 'iterator' to reduce memory consumed by join
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