如何在SQL Server 2008中完成查询执行

Question

I am using SQL Server 2008 and would really like to know how the engine executes queries.

Let's suppose I run this query :

select * from <table1> with(nolock)

or same with a join:

select * from <table1> a with(nolock) 
join <table2> b with(nolock) on a.<column1> = b.<column1> 
where <some condition>

How does the query work behind the scene?
Is there any way to see actual steps that are performed for execution and showing results?

Answer 1

The database engine analyses the query and selects the best way of retrieving the requested data under the current conditions.

See Displaying Graphical Execution Plans (SQL Server Management Studio) .

Answer 2

LISTING 1-1 Logical query processing step numbers

(5) SELECT (5-2) DISTINCT (5-3) TOP(<top_specification>) (5-1) <select_list> (1) FROM (1-J) <left_table> <join_type> JOIN <right_table> ON <on_predicate> | (1-A) <left_table> <apply_type> APPLY <right_table_expression> AS <alias> | (1-P) <left_table> PIVOT(<pivot_specification>) AS <alias>| (1-U) <left_table> UNPIVOT(<unpivot_specification>) AS <alias> (2) WHERE <where_predicate> (3) GROUP BY <group_by_specification> (4) HAVING <having_predicate> (6) ORDER BY <order_by_list>;

Logical Query Processing Phases in Brief

■ (1) FROM The FROM phase identifi es the query's source tables and processes table operators. Each table operator applies a series of subphases. For example, the phases involved in a join are (1-J1) Cartesian Product, (1-J2) ON Filter, (1-J3) Add Outer Rows. The FROM phase generates virtual table VT1.

■ (1-J1) Cartesian Product This phase performs a Cartesian product (cross join) between the two tables involved in the table operator, generating VT1-J1.

■ (1-J2) ON Filter This phase fi lters the rows from VT1-J1 based on the predicate that appears in the ON clause (). Only rows for which the predicate evaluates to TRUE are inserted into VT1-J2. (1-J3) Add Outer Rows If OUTER JOIN is specifi ed (as opposed to CROSS JOIN or INNER JOIN), rows from the preserved table or tables for which a match was not found are added to the rows from VT1-J2 as outer rows, generating VT1-J3.

■ (2) WHERE This phase fi lters the rows from VT1 based on the predicate that appears in the WHERE clause (). Only rows for which the predicate evaluates to TRUE are inserted into VT2.

■ (3) GROUP BY This phase arranges the rows from VT2 in groups based on the column list specifi ed in the GROUP BY clause, generating VT3. Ultimately, there will be one result row per group.

■ (4) HAVING This phase fi lters the groups from VT3 based on the predicate that appears in the HAVING clause (). Only groups for which the predicate evaluates to TRUE are inserted into VT4.

■ (5) SELECT This phase processes the elements in the SELECT clause, generating VT5.

■ (5-1) Evaluate Expressions This phase evaluates the expressions in the SELECT list, generating VT5-1.

■ (5-2) DISTINCT This phase removes duplicate rows from VT5-1, generating VT5-2.

■ (5-3) TOP This phase fi lters the specifi ed top number or percentage of rows from VT5-2 based on the logical ordering defi ned by the ORDER BY clause, generating the table VT5-3.

■ (6) ORDER BY This phase sorts the rows from VT5-3 according to the column list specifi ed in the ORDER BY clause, generating the cursor VC6.