Unique Key Index FAQS

 1. What is a Unique Key Index in Oracle?

• A Unique Key Index in Oracle is a type of index that ensures all values in the indexed column(s) are unique. It is created automatically when you define a UNIQUE constraint or a PRIMARY KEY constraint on a column, but can also be explicitly created using the CREATE INDEX statement.

2. How does a Unique Key Index work?

• A Unique Key Index enforces the uniqueness of values in the indexed column(s). When you attempt to insert or update a value, Oracle checks the index to ensure the value is unique in the table. If a duplicate value is found, it raises an error.

3. How do I create a Unique Key Index in Oracle?

• You can create a Unique Key Index automatically by defining a UNIQUE or PRIMARY KEY constraint on a table:

·        ALTER TABLE employees ADD CONSTRAINT unique_email UNIQUE (email);

• Alternatively, you can manually create a Unique Key Index using the CREATE UNIQUE INDEX statement:

·        CREATE UNIQUE INDEX idx_unique_email ON employees (email);

4. Can I create a Unique Key Index on multiple columns?

• Yes, you can create a composite Unique Key Index on multiple columns. This ensures that the combination of values across these columns is unique:

·        CREATE UNIQUE INDEX idx_unique_name_dob ON employees (first_name, last_name, dob);

5. What is the difference between a Unique Key Index and a Primary Key Index?

• Both Unique Key Index and Primary Key Index enforce uniqueness. However:
• A Primary Key does not allow NULL values, while a Unique Key can allow NULLs unless explicitly restricted by a NOT NULL constraint.
• A table can have only one Primary Key, but it can have multiple Unique Constraints.

6. Can a Unique Key Index allow NULL values?

• Yes, a Unique Key Index allows NULL values. In fact, Oracle treats NULLs as distinct values in unique constraints, so multiple rows with NULL values in a unique column are allowed.

7. What are the benefits of using a Unique Key Index?

• Prevents Duplicates: Ensures that no two rows in the table have the same value for the indexed column(s).
• Improves Query Performance: Speeds up queries that search for specific values in the indexed column(s) by providing efficient access to data.
• Supports Data Integrity: Helps enforce referential integrity when foreign keys reference unique columns in other tables.

8. What are the limitations of a Unique Key Index?

• Insert/Update Overhead: Every time you insert or update data in the indexed column, Oracle must check if the value violates the uniqueness rule, adding processing time.
• Storage Overhead: Indexes require additional disk space to store the index structure.
• Cannot have Duplicates (except NULLs): Ensures uniqueness, so you cannot insert or update data with duplicate values in the indexed column(s).

9. Can a Unique Key Index be created manually without using a constraint?

• Yes, you can create a Unique Key Index explicitly without using a constraint, like this:

·        CREATE UNIQUE INDEX idx_unique_email ON users (email);

10. Can I drop or rebuild a Unique Key Index?

• Yes, you can drop or rebuild a Unique Key Index just like any other index. For example:
• To drop an index:

o   DROP INDEX idx_unique_email;

• To rebuild an index:

o   ALTER INDEX idx_unique_email REBUILD;

11. Can I create multiple Unique Key Indexes on the same column?

• No, you cannot create multiple Unique Key Indexes on the same column. A Unique Constraint automatically creates a Unique Key Index for that column, and only one index can enforce uniqueness for the same column.

12. Does Oracle automatically create a Unique Key Index for a PRIMARY KEY?

• Yes, when you define a PRIMARY KEY constraint on a column, Oracle automatically creates a Unique Key Index on that column to enforce the uniqueness of the primary key.

13. What happens if I try to insert a duplicate value into a column with a Unique Key Index?

• If you try to insert a duplicate value into a column that has a Unique Key Index, Oracle will raise an error (e.g., ORA-00001: unique constraint (constraint_name) violated), preventing the insertion of the duplicate value.

14. Can I use a Unique Key Index with foreign keys?

• Yes, a Unique Key Index is often used with foreign keys. For example, a foreign key in one table can reference a column in another table that has a Unique Key Index, ensuring that the foreign key values are consistent with the unique values in the referenced table.

15. How does a Unique Key Index help in improving performance?

• A Unique Key Index speeds up query performance by allowing Oracle to quickly search and retrieve records based on the indexed column(s). It also helps in fast lookups for queries that check for the existence of unique values in the table.

16. What types of queries benefit from Unique Key Indexes?

• Queries that check for the existence of specific values in the indexed column(s), such as:
• Equality Searches: SELECT * FROM users WHERE email = 'example@example.com';
• Join Queries where the indexed column is used to match records across tables.

17. Can I use Unique Key Indexes for performance tuning?

• Yes, you can use Unique Key Indexes to improve performance by speeding up lookups and searches on unique columns. However, for queries that require range scans or sorting, a regular B-tree Index or other types of indexes may be more appropriate.

18. Can I use Unique Key Indexes for both OLTP and OLAP systems?

• OLTP systems (Transactional systems) often use Unique Key Indexes to enforce data integrity and to ensure fast lookups for user identification, email addresses, or other unique values.
• For OLAP systems (Analytical systems), the usage of Unique Key Indexes depends on the data model, but in general, these indexes can still help speed up queries for certain types of aggregations or lookups on unique columns.

19. Is it possible to create a Unique Key Index on non-numeric columns?

• Yes, Unique Key Indexes can be created on both numeric and non-numeric columns. For example, you can create a unique index on a VARCHAR column or a DATE column to ensure uniqueness for those types of data.

20. How do I monitor the usage of Unique Key Indexes?

• You can monitor the performance of Unique Key Indexes by using the following:
• EXPLAIN PLAN: To analyze how queries use the index.
• DBA_INDEXES: To examine index statistics and usage.
• SQL Trace: To capture query execution details and check the impact of indexes.