Here's an excerpt of this article:
SELECT id, name, address, phone FROM customers ORDER BY name LIMIT 10 OFFSET 990;
MySQL is first scanning an index then retrieving rows in the table by primary key id. So it’s doing double lookups and so forth.
The following piece just uses the primary key:
SELECT id FROM customers ORDER BY name LIMIT 10 OFFSET 990;
I don't figure out the difference between those two queries despite the explanation, especially the evoked double lookup...
May someone explain it in more detail?