Aggregate functions like Count, max, min, avg performance really depends on how did you tune your database for that kind of workload. Let us discuss this.
0:00 Intro
1:22 SELECT COUNT(*)
4:30 SELECT AVG(A)
5:15 SELECT MAX(A)
8:00 Best case scenario
11:30 Clustering
14:00 Clustering Sequential Writes
17:19 Clustering Random Writes
20:30 Summary