Unique and non-unique indexes
Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values.
When you create a unique index for an existing table with data, values in the columns or expressions that comprise the index key are checked for uniqueness. If the table contains rows with duplicate key values, the index creation process fails. When a unique index is defined for a table, uniqueness is enforced whenever keys are added or changed within the index. This enforcement includes insert, update, load, import, and set integrity, to name a few. In addition to enforcing the uniqueness of data values, a unique index can also be used to improve data retrieval performance during query processing.
Non-unique indexes are not used to enforce constraints on the tables with which they are associated. Instead, non-unique indexes are used solely to improve query performance by maintaining a sorted order of data values that are used frequently.
Including and excluding NULL keysUnique and non-unique indexes can be created so that a key is not inserted into the index object when all columns or expressions of the key are null. Excluding null keys can result in improved storage and performance optimization for cases where you do not want queries to access data associated with null keys. For unique indexes, the enforcement of the uniqueness of table data ignores rows where the index key is null.
Differences between primary key or unique key constraints and unique indexes
It is important to understand that there is no significant difference between a primary key or unique key constraint and a unique index. To implement the concept of primary and unique key constraints, the database manager uses a combination of a unique index and the NOT NULL constraint. Therefore, unique indexes do not enforce primary key constraints by themselves because they allow null values. Although null values represent unknown values, when it comes to indexing, a null value is treated as being equal to other null values.
Therefore, if a unique index consists of a single column, only one null value is allowed-more than one null value would violate the unique constraint. Similarly, if a unique index consists of multiple columns, a specific combination of values and nulls can be used only one time.