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.