How do you know when it's time to defrag your indexes? How much fragmentation is too much? Is it 20 percent, or maybe 30 percent? Unfortunately, there's no magic number, and average fragmentation percentage isn't the only factor to consider when determining whether you need to rebuild or reorganize an index. Check out these considerations to keep in mind, and walk through an example that illuminates the effect of a fragmented index.
The article continues at http://www.sqlmag.com/Articles/ArticleID/102460/102460.html?Ad=1