My understanding about SQL Server threads is that SQL Server breaks down an execution request within a session into several tasks and associates each task with a worker thread. So SQL Server always tries to use multiple threads (if available and needed ) to run a query, regardless of whether it is a parallel or nonparallel query.
But the following statement (From book Microsoft SQL Server 2012 Internals (MS Press)) seems to state something different:
A parallel query execution plan can use more than one thread; a serial execution plan, used by a nonparallel query, uses only a single thread.
Edit: My original understanding is (likely incorrect) a scheduler may (when there is a need) use multiple workers/threads from the same NUMA node to process different tasks of the same execution request when it is a serial execution.
For a parallel execution, multiple schedulers/processors would process the same execution request (which is broken down to several tasks) at the same time, and each of the scheduler will use (again, when there is a need) multiple workers/threads from their respective NUMA node.
In other words, parallel or not, there is a chance multiple threads are used.
Obviously there must be a flaw in my original understanding, but I don't know where the misconception is. Does my original understanding only apply when it is a PARALLEL query being executed?