Why and When do we need to Gather Statistics ?

Long back a customer raised a query "Why do I need to Gather Statistics ?".

There are many such questions that come up regularly. Some of these are :

"When I am OK with the current performance of a Query / Database with present statistics, why should I re-gather statistics and change these?"

"What should be the frequency of gathering statistics ?"

These queries, sometimes are obvious due to the reason that many a times, performance issues are reported after gathering of statistics.

Again, I would maintain that most of the times these are the issues related to the application queries, hence, proper optimizer statistics, optimal application design and a perfect indexing strategy plays an important role in the scalability of the system.

For optimal application design, I already have a blog posted on December 9, 2007 http://viveklsharma.blogspot.com/2007/12/optimizer-calculation-on-dependant.html

In this case, subquery factoring would have helped with the stability in plan, no matter what the stats are.

Anyways, all these will be answered in my next few blogs.