Some operations in SQL Server will turn your entire query plan serial (single-threaded), others will just reserve a so-called “serial zone”. I read up on this stuff a number of years ago (including a great post by Paul White), and thinking that some things must have changed since, I decided to go see for myself.
The subject of parallel execution of SQL Server queries is at times somewhat shrouded in mystery and uncertainty. Since the concept of parallel execution is such a significant (and indispensable tool) for performance tuning, it’s good to have a fair idea of how it works. In this first post in a series on parallelization, I’m going to try to sort out the apples from the pears, or the serial from the parallel if you will.