Serial Plans And Serial Zones

Daniel Hutmacher takes a look at what causes your T-SQL code to use a serial plan or at least go into a zone where parallelism is not an option:

Modifications to table variables

UPDATE, INSERT and DELETE on table variables cause a completely serial plan. SELECT statements, on the other hand, don’t neccessarily.

Scalar functions

Completely serial. Even when they’re used in computed columns in one of the tables. Even when you’re not referencing that actual column.

Not all computed columns generate serial plans – only those with scalar functions.

Read on for a number of other places.  It turns out that this set is pretty stable from 2012 through to 2017.