Indexes are the most important tool for query optimization. A well-designed index can turn a query from seconds to milliseconds. Here's how to use them effectively.
How Indexes Work
Without index (table scan):
Query: SELECT * FROM users WHERE email = 'john@example.com'
Table: 1,000,000 rows
Operation: Scan every row → 1,000,000 comparisons
Time: O(n)
With index (index lookup):
B-tree index on email column
Operation: Tree traversal → ~20 comparisons
Time: O(log n)
Index Types
B-Tree (Default)
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Hash Index
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GIN (Generalized Inverted Index)
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GiST (Generalized Search Tree)
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Composite Indexes
Column Order Matters
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Design Principles
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Covering Indexes (Index-Only Scans)
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Partial Indexes
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Expression Indexes
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Analyzing Index Usage
EXPLAIN ANALYZE
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Finding Unused Indexes
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Finding Missing Indexes
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Common Patterns
Foreign Key Indexes
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Pagination
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Soft Deletes
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Anti-Patterns
Over-Indexing
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Low Selectivity Indexes
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Functions Preventing Index Use
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Maintenance
Monitoring Index Bloat
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Rebuilding Indexes
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Conclusion
Indexes are essential for query performance but come with trade-offs. Start with indexes on foreign keys and frequently-queried columns. Use EXPLAIN ANALYZE to verify index usage. Monitor for unused indexes and remove them.
Remember: the best index is one that helps your actual queries. Profile first, then optimize.