Plansplaining, part 1. The unexpected aggregation and assert
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When I look at an execution plan I sometimes right away see how the operators work together. Other times I need to dive into the details and put in effort before I really understand it. And occasionally, I think I understand but then am proven wrong after I start looking at the details. However, understanding all the details of an execution plan is really important when you want to optimize performance. If I see an execution plan where I do not understand the role of each and every operator, I know I do not truly understand how the query is…
