Query Tuning Essentials: What Forces SQL Server Queries To Run Single Threaded?
Going Further
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Would also add that prior to 2019, UDFs on computed columns prevent queries from going parallel, even if you don’t reference the column in your query!
(Sorry if you covered this and I missed it!).
Yep, I cover that. Also, UDF inlining doesn’t fix that issue at all. It’s explicitly called out in the documentation.