Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/7417#issuecomment-145190616
@zsxwing the rdds order do matters for `RDD.cartesian`, because of the
inefficient way we compute `CartesianRDD`:
```
for (x <- rdd1.iterator(currSplit.s1, context);
y <- rdd2.iterator(currSplit.s2, context)) yield (x, y)
```
For `a.cartesian(b)`, when we compute partition in result RDD, logically we
only need to compute the referenced partition of `a` and `b` once, but actually
we comput partition of `b` many times according to the number of elements in
partition of `a`.
Anyway this PR LGTM
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