Common Query Optimization Problems
So you've written the query and examined the query plan, and performance isn't what you expected. It might appear that SQL Server isn't choosing the appropriate query plan that you expect. Is something wrong with the query or with the optimizer? Before we delve into a detailed discussion about how to debug and analyze query plans (covered in detail in the next chapter), this section takes a look at some of the more common problems and SQL-coding issues that can lead to poor query plan selection.
Out-of-Date or Unavailable Statistics
Admittedly, this is not as big a problem as it was in SQL Server releases prior to 7.0. Back in those days, the first question asked when someone was complaining of poor performance ...