What's a covering index and why is it faster than a normal index?
A covering index includes every column a query needs, directly in the index itself. The database can answer the query by reading only the index and never touching the actual table rows — skipping an extra disk lookup per matching row. The trade-off is a larger index that costs more to store and update on every write.
Answered in
Database Indexing Explained: Which Index Type Actually Fits Your QueryB-tree, hash, composite, and covering indexes each solve a different query shape. Pick the wrong one and you pay index overhead without the speedup.
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