»If at all possible, we build databases so that the working set of the database fits into memory.« What does that even mean?
In computer science, the “Working Set” of a program is the set of things it will be accessing in the near future. Because computer science has not yet solved looking into the future, we are looking at the set of things we accessed most recently and hope for The Best™.
The Best™ here being that our future access pattern is very similar to our most recent past access pattern.
In MySQL, the “things we access” in the Working Set definition is RAM, Memory.
We do care, because memory is fast. How fast is fast?
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