How does the TOP, DESC and CROSS APPLY make this query faster?

Question

Related to this question: In what order does execution on WHERE and ON clauses work?

I was reading this page about APPLY:

http://sqlblog.com/blogs/alexander_kuznetsov/archive/2009/07/07/using-cross-apply-to-optimize-joins-on-between-conditions.aspx

And I don't understand how the CROSS APPLY, TOP, and DESC are making this query faster.

Original query:

 SELECT s.StartedAt, s.EndedAt, c.AirTime 
FROM dbo.Commercials s JOIN dbo.Calls c  
  ON c.AirTime >= s.StartedAt AND c.AirTime < s.EndedAt 
WHERE c.AirTime BETWEEN '20080701' AND '20080701 03:00'

Faster query:

SELECT s.StartedAt, s.EndedAt, c.AirTime 
FROM dbo.Calls c CROSS APPLY( 
  SELECT TOP 1 s.StartedAt, s.EndedAt FROM dbo.Commercials s  
  WHERE c.AirTime >= s.StartedAt AND c.AirTime < s.EndedAt 
  ORDER BY s.StartedAt DESC) AS s 
WHERE c.AirTime BETWEEN '20080701' AND '20080701 03:00'

I don't know what specific question to ask, because I don't get it.

Answer 1

Your second query is likely faster because you are limiting the result set to join against to a maximum of 1 row; the first row. There is little to no computation or join matching required for that. In this case, your CROSS APPLY is acting like a singular function.