Github user steveloughran commented on a diff in the pull request:
https://github.com/apache/spark/pull/11242#discussion_r60199645
--- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala ---
@@ -62,8 +62,14 @@ class UnionRDD[T: ClassTag](
var rdds: Seq[RDD[T]])
extends RDD[T](sc, Nil) { // Nil since we implement getDependencies
+ // visible for testing
+ private[spark] val isPartitionEvalParallel: Boolean =
+ rdds.length > conf.getInt("spark.rdd.parallelListingThreshold", 10)
--- End diff --
Changing a config option is turning on a different codepath; in theory it
should go through a QE process. At least with a config option, when it does
play up, you can roll back by changing the switch and restarting: you've saved
the cost of redeploying the old code.
Undocumented options are dangerous though
* nobody in support will know what to do if problems surface and the switch
is set
* someone is going to find it in the code and wonder "what does this do"...
the origins will be lost in the folklore of the project.
What about having a section in the docs on "unstable+unsupported options",
list it with a link to the JIRA and a "use at your own risk" disclaimer?
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