title: What does MySQl explain Extra "Using where" really mean?
body: <p>According to the <a href="https://dev.mysql.com/doc/refman/8.0/en/explain-output.html#explain-extra-information" rel="nofollow noreferrer">MySQL documentation</a>, Using where means: A WHERE clause is used to restrict which rows to match against the next table or send to the client.</p>
  <p>As I understand, it means if your sql statement has a where condition, a 'Using where' will appear in your explain Extra information. While as my experience, it seems mean MySQL storage engine finds that the index  couldn't cover some column, and will have to retrieve row data. For example:</p>
  <p><a href="https://i.stack.imgur.com/alpfi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/alpfi.png" alt="enter image description here" /></a></p>
  <p>Could anyone explain the real meaning of Using where?</p>