The original title is "Database Indexes: What's Actually Happening Under the Hood"

Short summary

A detailed walkthrough of how database indexes work internally, covering B+Tree structure, selectivity, composite index leftmost-prefix rules, and practical guidance on when to index. The article explains why the optimizer sometimes ignores indexes and how function-wrapped columns break index usage. It closes with trade-offs for write-heavy transactional tables.

• •B+Trees are the default index structure because they handle ranges, sorting, and prefix matching unlike hash indexes
• •Selectivity (distinct values / total rows) determines whether an index is useful; low-selectivity columns often result in full scans
• •Composite indexes follow a leftmost-prefix rule; wrapping columns in functions defeats index usage

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