In what order does execution on WHERE and ON clauses work?

Question

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

Answer 1

There is no definitive "before" and "after" on these queries. RDBMS is allowed to decide when to run what part of the query, as long as the results the query produces do not change.

In the first case, there is nothing the query can do to pre-filter the rows of Commercials, because the WHERE clause constrains only the rows of the Calls. These constraints specified a range for c.AirTime in terms of the corresponding row of Commercials, so no pre-filtering is possible: all rows of Calls would be considered for each row of Commercials.

In the second case, however, RDBMS can improve on the time by observing that you additionally constraint the range for c.AirTime to between 23:45 on Jun-30, 2008 through midnight of Jul-1, 2008 by constraining s.StartedAt to which c.AirTime is joined. This can allow the optimizer use an index, if one is defined on the Calls.AirTime column.

The important observation here is that the RDBMS can do very clever things when optimizing your query. It arrives at the optimized strategy by applying multiple rules of logic, trying to push the constraints closer to the "source of rows" in a join. The best option to checking what the optimizer does is reading the query plan.