Max Degree of Parallelism

Complex/Long running queries – During query optimization, SQL Server looks for queries that might benefit from parallel execution. It distinguishes between queries that benefit from parallelism and those that do not benefit, by comparing the cost of an execution plan using a single processor versus the cost of an execution plan using more than one processor and uses the cost threshold for parallelism (By default 5, this value of cost threshold for parallelism determines which queries are considered short, and they should therefore be run using serial plans) value as a boundary point to determine short or long query. In a parallel query execution plan, the INSERT, UPDATE, and DELETE operators are executed serially. However, the WHERE clause of an UPDATE or a DELETE statement, or the SELECT part of an INSERT statement may be executed in parallel. The actual data changes are then serially applied to the database.  Index data definition language (DDL) – Index operations that create or rebuild an index (REBUILD only, not applicable to REORGANIZE), or drop a clustered index and queries that use CPU cycles heavily are the best candidates for a parallel plan. For example, joins of large tables, large aggregations, and sorting of large result sets are good candidates.   Other Operations – Apart from the above, this option also controls the parallelism of DBCC CHECKTABLE, DBCC CHECKDB, and DBCC CHECKFILEGROUP.

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