How Database Indexes Actually Work and When to Use Them

Database indexes are a common tool for backend developers, but few understand what the engine does with them under the hood. A B-tree index allows the database to navigate data like a sorted tree, avoiding full table scans that check every row. Developers can use EXPLAIN to inspect query execution, where a type of ALL combined with a high row count signals a missing or unused index. Common pitfalls include wrapping indexed columns in functions, such as YEAR(created_at), which forces the engine to evaluate every row and bypasses the index entirely. For write-heavy systems like payment platforms, indexing trade-offs are especially significant since every index added also slows down insert and update operations.