When either a heap or an index is being accessed, internally there’s an object called a HeapDataSetSession or IndexDataSetSession, respectively. When a parallel scan is being performed, the threads doing the actual work of the scan each have a “child” dataset (another instance of the two objects I just described), and the main dataset, which is really controlling the scan, is called the “parent.”
When one of the scan worker threads has exhausted the set of rows it’s supposed to scan, it needs to get a new range by accessing the parent dataset, which means acquiring the ACCESS_METHODS_DATASET_PARENT latch in exclusive mode. While this can seem like a bottleneck, it’s not really, and there’s nothing you can do to stop the threads performing a parallel scan from occasionally showing a LATCH_EX wait for this latch.
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