Question

In the book Inside Microsoft SQL Server™ 2005 T-SQL Querying, the author Itzik Ben-Gan tell us the order or SQL Server 2005’s logical query processing is:

(8)  SELECT (9) DISTINCT (11) <TOP_specification> <select_list>
(1)  FROM <left_table>
(3)       <join_type> JOIN <right_table>
(2)       ON <join_condition>
(4)  WHERE <where_condition>
(5)  GROUP BY <group_by_list>
(6)  WITH {CUBE | ROLLUP}
(7)  HAVING <having_condition>
(10) ORDER BY <order_by_list>

1.  FROM
2.  ON
3.  OUTER (join)
4.  WHERE
5.  GROUP BY
6.  CUBE | ROLLUP
7.  HAVING
8.  SELECT
9.  DISTINCT
10. ORDER BY  <---------------------- NOTE
11. TOP       <---------------------- NOTE

In his book Inside Microsoft SQL Server 2008 : T-SQL Querying, he tell us the following logical query processing order:

(1) FROM
(1-J1) Cartesian Product
(1-J2) ON Filter
(1-J3) Add Outer Rows
(2) WHERE
(3) GROUP BY
(4) HAVING
(5) SELECT
(5-1) Evaluate Expressions
(5-2) DISTINCT
(5-3) TOP       <---------------------- NOTE
(6) ORDER BY    <---------------------- NOTE

Note the order of TOP and ORDER BY in the upper excerpts from these books. They are just opposite. I think these two step is very important, and will give a totally different result with different order. I want to know whether SQL Server 2008 changed something in it’s storage engine from SQL Server 2005 or something else reason cause this?

Thanks.

Answer 1

The logical processing order is also documented in this Books Online entry. Be careful to distinguish logical processing order from physical processing order. As the BOL entry notes:

The following steps show the logical processing order, or binding
order, for a SELECT statement. This order determines when the objects
defined in one step are made available to the clauses in subsequent
steps. For example, if the query processor can bind to (access) the
tables or views defined in the FROM clause, these objects and their
columns are made available to all subsequent steps. Conversely,
because the SELECT clause is step 8, any column aliases or derived
columns defined in that clause cannot be referenced by preceding
clauses. However, they can be referenced by subsequent clauses such as
the ORDER BY clause. Note that the actual physical execution of the
statement is determined by the query processor and the order may vary
from this list.

The query optimizer is free to translate the logical requirement specified by the query into any physical execution plan that produces the correct results. Generally, there are many physical alternatives for a given logical query, so it is quite usual for a physical plan to differ fundamentally from the logical processing order (for binding purposes) described above.