If the query involves a join, and max scan parallel degree is set to 3, and the nonpartitioned heap table is the outer table in the query, then the optimizer considers the following access methods:
A hash-based table scan using 3 worker processes
A range scan using 10 worker processes for the merge join
A serial scan using a single process
If max scan parallel degree is set to 1, then the optimizer does not consider the hash-based scan.
See “Single-table scans” for more examples of determining the degree of parallelism for queries.