What is the difference between Oracle unique indexes and regular indexes?

There are differences between unique indexes and regular indexes in Oracle.

1. Uniqueness: A unique index ensures that the values in the indexed column are unique, meaning each index key value can only appear once in the table. In contrast, a regular index does not have this restriction and allows for duplicate values in the indexed column.
2. Constraint: Unique index can be used as a unique constraint, which means when creating an index, you can specify it as UNIQUE. This will result in an error if a unique constraint is violated during data insertion or update. Regular indexes do not have this constraint.
3. Performance in querying: Due to the uniqueness constraint of a unique index, when searching for a unique index, the database can stop searching after finding the first matching index key value, thereby improving query performance. Regular indexes, on the other hand, require searching for all matching index key values.
4. Null values: Unique indexes can include null values, which means a NULL value. In a unique index, multiple null values are allowed, but only one non-null value. Normal indexes can also include null values.
5. Composite Index: A unique index can be a composite index, consisting of multiple columns. In this way, the uniqueness constraint will apply to all columns in the index. A regular index can also be a composite index.

It is important to consider the specific business requirements when deciding whether to use a unique index or a regular index to maintain data consistency and integrity.