Mapping between performance counter and SQL Server wait type.

What selectivity is really means ?

• It’s not about how unique each row is by itself.
• It’s about your query, and how small a percentage of the table you’re searching for.
• When evaluating column order for indexes, don’t think about how unique each column is. Think about what percentage you’re searching for.
  (from Brent Ozar - Fundamental Index Tuning)

updating ..

Have you ever thought that your SQL server takes too long to start up? Or when using SQL Availability Group and switching to another node for maintenance, did it also cause lengthy downtime? I’ve been there! So, in this post, I want to explore what’s causing these problems.