getPreferredLocations(partition: Partition): Seq[String]
StateStoreAwareZipPartitionsRDD is created exclusively when
StreamingSymmetricHashJoinExec physical operator is requested to execute and generate a recipe for a distributed computation (as an RDD[InternalRow]) (and requests StateStoreAwareZipPartitionsHelper for one).
StateStoreAwareZipPartitionsRDD takes the following to be created:
getPreferredLocations simply requests the StateStoreCoordinatorRef for the location of every state store (with the StatefulOperatorStateInfo and the partition ID) and returns unique executor IDs (so that processing a partition happens on the executor with the proper state store for the operator and the partition).