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Parallelism and Nested Loops Joins

Erik Darling talks about the intersection of two performance tuning topics:

Yesterday we saw a case where the Gather Streams operator was costed quite highly, and it prevented a parallel plan from being chosen, despite the parallel plan in this case being much faster.

It’s important to note that costing for plans is not a direct reflection of actual time or effort, nor is it accurate to your local configuration.

They’re estimates used to come up with a plan. When you get an actual plan, there are no added-in “Actual Cost” metrics.

Read on to see how you can monkey’s paw your way through this problem by introducing exciting, new problems.