Sams Teach Yourself SQL in 24 Hours, Second Edition

When Should Indexes Be Avoided?

Although indexes are intended to enhance a database's performance, there are times when they should be avoided. The following guidelines indicate when the use of an index should be reconsidered:

• Indexes should not be used on small tables.

• Indexes should not be used on columns that return a high percentage of data rows when used as a filter condition in a query's WHERE clause. For instance, you would not have an entry for the word "the" or "and" in the index of a book.

• Tables that have frequent, large batch update jobs run can be indexed. However, the batch job's performance is slowed considerably by the index. The conflict of having an index on a table that is frequently loaded or manipulated by a large batch process ...