SQL Anywhere automatically indexes primary key and foreign key columns. So, manually creating an index on a key column is not necessary and is generally not recommended. If a column is only part of a key, an index can help.

Indexes require extra space and can slightly reduce the performance of statements that modify the data in the table, such as INSERT, UPDATE, and DELETE statements. However, they can improve search performance dramatically and are highly recommended whenever you search data frequently. To learn more about how indexes improve performance, see Using indexes.

The optimizer automatically uses indexes to improve the performance of any database statement whenever it is possible to do so. Also, the index is updated automatically when rows are deleted, updated, or inserted. While you can explicitly refer to indexes using index hints when forming your query, there is no need to.