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Database Essentials
Database Design : Defining indexes : Choosing which tables and columns to index

Choosing which tables and columns to index

If you perform frequent adds, deletes, and updates against a small table, you might not want to index it because of slower performance caused by the index overhead. However, if you mostly perform retrievals, then it is useful to create an index for the table. You can index the columns that are retrieved most often, and in the order they are most often retrieved.
You do not have to create an index if you are retrieving a large percentage of the rows in your database (for example, 19,000 out of 20,000 rows) because it is more efficient to scan the table. However, it is worth the effort to create an index to retrieve a very small number of rows (for example, 100 out of 20,000) because then OpenEdge only scans the index table instead of the entire table.