Physical Database Design

58 CHAPTER 4 Selecting Indexes

that most DBMSs sort the data using more efficient algorithms, and then build the

actual index.

Rule 10. Avoid extremes in index cardinality and value distribution. Database

users with experience in performance tuning often find that they need to avoid

index distributions that are extremely skewed. Don’t allow just a few index values to

have much higher levels of frequency than other values. When this occurs, you

need to redesign the index to get a more balanced distribution and avoid costly

CPU and I/O delays for certain types of updates.

Also, you want to avoid extremes in index cardinality. For example, you want to

avoid indexes with only one entry, particularly for tables with over 1,000 rows. You

also want to avoid ...