Count the trips to the shelf, not the glances at the page.

A database fetches data in fixed-size pages, each holding many rows. Once a page is in memory, checking the values on it costs almost nothing. The slow, costly step is bringing another page in from disk. So when you judge a query, ask how many pages it must fetch; an index pays off by shrinking that number.

Read it once, then a question tests whether the trick stuck.