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Logical Reads and Query Tuning

Erik Darling doesn’t focus on logical reads:

To summarize the video a little bit:

  • High average or total logical reads isn’t a guarantee that a query is slow
  • Looking for high average CPU and duration queries is a better metric
  • You may see logical reads go up or down as you make queries faster
  • For I/O bound workloads, you’re better off looking for queries with a lot of physical reads

I agree with Erik. Disk has gotten so much faster, especially if you’re on all-flash arrays or (even better) direct attached nVME storage. Even relatively old SSDs are still a couple orders of magnitude faster than the spinning rust we typically dealt with 15 years ago.

Sometimes, the faster query is one that requires more logical reads. APPLY-based queries typically fall into that category: I might have 10x as many logical reads but the query takes half the time (or less) to finish using a similar percentage reduction of CPU time.