Github user mengxr commented on a diff in the pull request: https://github.com/apache/spark/pull/21927#discussion_r207107889 --- Diff: core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala --- @@ -340,6 +340,22 @@ class DAGScheduler( } } + /** + * Check to make sure we don't launch a barrier stage with unsupported RDD chain pattern. The + * following patterns are not supported: + * 1. Ancestor RDDs that have different number of partitions from the resulting RDD (eg. + * union()/coalesce()/first()/PartitionPruningRDD); --- End diff -- `coalesce()` is not safe when shuffle is false because it may cause the number of tasks doesn't match the number of partitions for the RDD that uses barrier mode.
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