If you most often insert, update, and delete rows, then the fewer indexes associated with the table, the better the performance. This is because the driver must maintain the indexes as well as the database tables, thus slowing down the performance of row inserts, updates, and deletes. It may be more efficient to drop all indexes before modifying a large number of rows, and re-create the indexes after the modifications.
If you most often retrieve rows, you must look further to define the criteria for retrieving rows and create indexes to improve the performance of these retrievals. Assume you have an employee database table and you will retrieve rows based on employee name, department, or hire date. You would create three indexes—one on the dept field, one on the hire_date field, and one on the last_name field. Or perhaps, for the retrievals based on the name field, you would want an index that concatenates the last_name and the first_name fields (see
“Indexing Multiple Fields” for details).