What Is a Unique and Primary Indexes in MySQL?
Primary keys are unique and non-null; InnoDB uses the primary key as clustered row storage. Unique secondary keys enforce distinct key combinations.
How the Unique and Primary Indexes Algorithm Works
Primary keys are unique and non-null; InnoDB uses the primary key as clustered row storage. Unique secondary keys enforce distinct key combinations.
When to Use Unique and Primary Indexes
Stable row identity and business uniqueness, including tenant-scoped combinations.
How to Create a MySQL Unique and Primary Indexes
Review data quality, existing structures, feature support, build time, and deployment requirements before creating an index on a production table.
CREATE UNIQUE INDEX users_tenant_email_uq ON users (tenant_id, email);Unique and Primary Indexes Query Example
This example demonstrates the matching query shape. Check the execution plan to confirm that the engine can use the index.
SELECT id FROM users WHERE tenant_id = 7 AND email = 'reader@example.com';Queries Supported by Unique and Primary Indexes
Stable row identity and business uniqueness, including tenant-scoped combinations. 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; wide primary keys enlarge every secondary index. Indexes reduce some search work but require storage and add maintenance to writes.
Unique and Primary Indexes Limitations and Common Mistakes
NULL, collation, and unique-combination rules must match intended identity semantics.
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 and Primary Indexes
Clean duplicates, define uniqueness scope, and keep the primary key narrow and stable.
Compare representative plans and timings before and after, including concurrent writes and realistic data volumes.
Unique and Primary 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 MySQL Index Documentation
Check MySQL 8.4 Reference Manual: Optimization and Indexes for release-specific requirements and supported operators.