What is a Unique Index? Ensuring Data Integrity with Fast Lookups

• What is a Unique Index?
• Why Use a Unique Index?
• Performance Considerations
• Use Cases
• Example

What is a Unique Index?

A Unique Index guarantees that all values in the indexed column or columns are unique—no duplicates allowed. It enforces a uniqueness constraint to keep data accurate and consistent.

Databases automatically create unique indexes for primary keys and UNIQUE constraints, but you can also add extra unique indexes on columns like emails or usernames.

Why Use a Unique Index?

Unique indexes help:

• Prevent duplicate data (e.g., no two users with the same email)
• Speed up queries on unique columns
• Allow the database to optimize queries since it expects only one matching row

Performance Considerations

Unique indexes make SELECT queries fast and efficient. However, inserts or updates on these columns carry a small overhead because the database must check for duplicates before allowing changes. This extra step ensures data integrity but slightly slows down writes.

Storage and maintenance costs are similar to regular indexes, making unique indexes a good trade-off for data correctness.

Use Cases

Use unique indexes for any column that must have distinct values, such as:

• Usernames
• Email addresses
• Social Security numbers
• Candidate keys apart from the primary key

Unlike the primary index, you can have multiple unique indexes per table.

Example

Here's how to create a unique index on an email column in a Users table:

CREATE UNIQUE INDEX idx_users_email ON Users(Email);

This ensures emails are unique, speeding up queries like:

SELECT * FROM Users WHERE Email = 'alice@example.com';

and preventing duplicate emails during inserts or updates.

A unique index is key for keeping your data clean and queries fast on columns that require unique values.