Queries that do not select any table columns - do I understand this correctly?

I just read this article:

http://use-the-index-luke.com/sql/clustering/index-only-scan-covering-index

And at the bottom is this statement:

Queries that do not select any table columns are often executed as index-only scan.

Can you think of a meaningful example?

Problem is, there is no comments section, so I just want to verify, this is one example, correct?

SELECT 1 FROM `table_name` WHERE `indexed_column` = ?

This is to check whether a specified row exists.

So the questions:

• Are there any more practical uses for that?

• As a side note, I read somewhere that the above query might be more performant 开发者_高级运维if encapsulated in EXISTS, I'm not sure how to check if it's true:

  SELECT EXISTS(SELECT 1 FROM `table_name` WHERE `indexed_column` = ? LIMIT 1)
  

  Is it?

Well, possibly the canonical example would be select count(*) from mytable to get a row count.

That selects no data from the table and would most likely be satisfied by the primary key index, if available.