Proper indexing is the single most impactful optimization you can make to a MySQL database. Without appropriate indexes, even simple queries can result in full table scans that bring your application to a crawl.
Index Types and Use Cases
B-tree indexes are the default and work well for equality and range queries. Composite indexes covering multiple columns should be designed with the most selective column first. The order of columns in a composite index directly affects which queries can benefit from it.
Use the EXPLAIN statement to analyze query execution plans and identify missing indexes. Look for "Using filesort" and "Using temporary" in the output, as these indicate operations that could benefit from better indexing strategies.
Be cautious about over-indexing. Every index adds overhead to INSERT, UPDATE, and DELETE operations because the index must be maintained alongside the data. Monitor your slow query log regularly and add indexes only where they provide measurable improvement.