What Is a Unique Indexes in Microsoft SQL Server?
Ensures indexed keys are distinct and can enforce business rules independently of primary-key status.
How the Unique Indexes Algorithm Works
Ensures indexed keys are distinct and can enforce business rules independently of primary-key status.
When to Use Unique Indexes
Concurrent-write-safe uniqueness for identifiers and combinations such as tenant plus email.
How to Create a Microsoft SQL Server Unique Indexes
Review data quality, existing structures, feature support, build time, and deployment requirements before creating an index on a production table.
CREATE UNIQUE INDEX UX_Users_TenantEmail ON dbo.Users (TenantId, Email);Unique Indexes Query Example
This example demonstrates the matching query shape. Check the execution plan to confirm that the engine can use the index.
SELECT UserId FROM dbo.Users WHERE TenantId = 7 AND Email = N'reader@example.com';Queries Supported by Unique Indexes
Concurrent-write-safe uniqueness for identifiers and combinations such as tenant plus email. A similar-looking predicate may not use the same access path; operators, expressions, key order, and data type all matter.
Read Performance, Storage, and Write Costs
Uniqueness checks add write work; creation fails if existing rows violate the rule. Indexes reduce some search work but require storage and add maintenance to writes.
Unique Indexes Limitations and Common Mistakes
NULL, collation, filtered uniqueness, and computed-key semantics affect exact rules.
Do not assume that an index being present means the optimizer chose it or that the query became selective.
How to Measure and Tune Unique Indexes
Clean duplicates and verify that the unique key has the correct scope.
Compare representative plans and timings before and after, including concurrent writes and realistic data volumes.
Unique Indexes Questions
Will the database always use this index?
No. The optimizer weighs selectivity, estimated costs, statistics, cache state, and the amount of data returned.
What should I verify before production?
Confirm version support, query compatibility, write overhead, build/rollback procedures, and index behavior during backup and recovery.
Official Microsoft SQL Server Index Documentation
Check Microsoft Learn: SQL Server Indexes for release-specific requirements and supported operators.