Backend & systems·Data modelling and storage engines
you add the extra column to the index so the query is answered from the index alone and never touches the table.
Covering index
Also calledindex-only scan, included columns
An index containing every column a query needs, so the engine can skip the lookup back into the heap or clustered rows. It is often the difference between a fast query and a fast query that still does a million random reads. The trade is index size and write cost, both of which grow with every column you include.