Database indexing is one of the highest ROI performance optimizations any backend developer can perform. Knowing which columns to index—and which to leave unindexed—separates junior engineers from seasoned architects.
Understanding Composite Index Left-Prefix Rule
When you create a composite index on (status, published_at), MySQL can use it for queries filtering by status or both status AND published_at. However, it cannot use this index if you only filter by published_at.
Covering Indexes
A covering index is an index that includes all the fields needed to satisfy a query. When MySQL uses a covering index, it does not need to look up data in the clustered index (table data), leading to 10x throughput increases.
