Keys and unique columns   SQL Anywhere automatically creates indexes on primary keys, foreign keys, and unique columns. You should not create additional indexes on these columns. The exception is composite keys, which can sometimes be enhanced with additional indexes.

For more information, see Composite indexes.

Frequency of search   If a particular column is searched frequently, you can achieve performance benefits by creating an index on that column. Creating an index on a column that is rarely searched may not be worthwhile.

Size of table   Indexes on relatively large tables with many rows provide greater benefits than indexes on relatively small tables. For example, a table with only 20 rows is unlikely to benefit from an index, since a sequential scan would not take any longer than an index lookup.

Number of updates   An index is updated every time a row is inserted or deleted from the table and every time an indexed column is updated. An index on a column slows the performance of inserts, updates and deletes. A database that is frequently updated should have fewer indexes than one that is read-only.

Space considerations   Indexes take up space within the database. If database size is a primary concern, you should create indexes sparingly.

Data distribution   If an index lookup returns too many values, it is more costly than a sequential scan. SQL Anywhere does not make use of the index when it recognizes this condition. For example, SQL Anywhere would not make use of an index on a column with only two values, such as Employees.Sex in the SQL Anywhere sample database. For this reason, you should not create an index on a column that has only a few distinct values.