MySQL Performance Tuning: Expert Guide to Optimizing Your Database Indexes

Why MySQL Index Optimization Matters

Proper indexing is the difference between sub-second queries and database timeouts. A well-optimized MySQL database with strategic indexing can handle 10x more concurrent users while reducing server costs by up to 60%. This comprehensive guide covers everything from basic index concepts to advanced optimization strategies used by Fortune 500 companies.

MySQL Index Types and When to Use Them

Primary Index (Clustered)

Automatically created with PRIMARY KEY. Data is physically sorted by this index.


              CREATE TABLE users (

                id INT PRIMARY KEY AUTO_INCREMENT,

                email VARCHAR(255)

              );

Secondary Index

Non-clustered indexes that point to primary key. Essential for fast lookups.


              CREATE INDEX idx_email

              ON users (email);

Composite Index

Multi-column indexes. Order matters - most selective column first.


              CREATE INDEX idx_user_status

              ON orders (user_id, status, created_at);

Covering Index

Includes all columns needed by query. Eliminates table lookups.


              CREATE INDEX idx_covering

              ON products (category_id, price, name);

Advanced Index Optimization Techniques

1. Index Selectivity Analysis

Create indexes on columns with high selectivity (unique values). Low selectivity indexes can hurt performance.


            -- Check selectivity of a column

            SELECT COUNT(DISTINCT email) / COUNT(*) as selectivity

            FROM users;


            -- Good selectivity: > 0.1 (10% unique values)

            -- Poor selectivity: < 0.01 (1% unique values)

Performance Impact: Proper selectivity analysis can reduce query time by 70%

2. Prefix Index Optimization

For VARCHAR columns, use prefix indexes to save space while maintaining performance.


            -- Find optimal prefix length

            SELECT

              LEFT(email, 5) as prefix,

              COUNT(*) as cnt

            FROM users

            GROUP BY LEFT(email, 5)

            ORDER BY cnt DESC;


            -- Create prefix index

            CREATE INDEX idx_email_prefix ON users (email(10));

3. Index Maintenance and Monitoring

Regular maintenance ensures indexes remain effective. Monitor index usage and fragmentation.


            -- Check index usage

            SELECT

              OBJECT_SCHEMA,

              OBJECT_NAME,

              INDEX_NAME,

              COUNT_FETCH,

              COUNT_INSERT,

              COUNT_UPDATE,

              COUNT_DELETE

            FROM performance_schema.table_io_waits_summary_by_index_usage

            WHERE OBJECT_SCHEMA = 'your_database'

            ORDER BY COUNT_FETCH DESC;

Common MySQL Indexing Mistakes to Avoid

Over-indexing

Too many indexes slow down INSERT/UPDATE operations and consume excessive storage.

Wrong Column Order

In composite indexes, placing less selective columns first reduces effectiveness.

Ignoring Query Patterns

Creating indexes without analyzing actual query usage patterns.

Unused Indexes

Keeping indexes that are never used by queries wastes resources.

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MySQL Performance Tuning: Complete Guide to Index Optimization