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Logical Query Processing Phases
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This section introduces the phases involved in the logical processing of a query. I ll rst brie y describe each step. Then, in the following sections, I ll describe the steps in much more detail and apply them to a sample query. You can use this section as a quick reference whenever you need to recall the order and general meaning of the different phases.
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Logical Query Processing
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Listing 1-1 contains a general form of a query, along with step numbers assigned according to the order in which the different clauses are logically processed.
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LISTING 1-1 Logical query processing step numbers
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(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>;
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Figure 1-1 contains a ow diagram representing logical query processing phases in detail. Throughout the chapter I ll refer to the step numbers that appear in the diagram. The rst noticeable aspect of SQL that is different from other programming languages is the order in which the code is processed. In most programming languages, the code is processed in the order in which it is written. In SQL, the rst clause that is processed is the FROM clause, while the SELECT clause, which appears rst, is processed almost last. Each step generates a virtual table that is used as the input to the following step. These virtual tables are not available to the caller (client application or outer query). Only the table generated by the nal step is returned to the caller. If a certain clause is not speci ed in a query, the corresponding step is simply skipped. The following section brie y describes the different logical steps.
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Logical Query Processing Phases in Brief
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Don t worry too much if the description of the steps doesn t seem to make much sense for now. These are provided as a reference. Sections that come after the scenario example will cover the steps in much more detail.
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(1) FROM The FROM phase identi 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 lters the rows from VT1-J1 based on the predicate that appears in the ON clause (<on_predicate>). Only rows for which the predicate evaluates to TRUE are inserted into VT1-J2.
Inside Microsoft SQL Server 2008: T-SQL Querying
Start
1 Entering FROM
First Table Operator Exist
Yes Operator Type
JOIN CROSS CROSS
1-J1 Cartesian Product 1-J2 ON Filter 1-J3 Add Outer Rows
APPLY
1-A1 Apply Table Expression 1-A2 Add Outer Rows
PIVOT
1-P1 Group 1-P2 Spread 1-P3 Aggregate
UNPIVOT
1-U1 Generate Copies 1-U2 Extract Element 1-U3 Remove NULLs
OUTER
INNER
OUTER
Another Table Operator Exists
No Yes
WHERE Exists 2 WHERE
No Yes
GROUP BY Exists 3 GROUP BY
No Yes
HAVING Exists 4 HAVING
5 Entering SELECT 5-1 Evaluate Expressions
DISTINCT Exists 5-2 DISTINCT
No Yes
TOP Exists 5-3 TOP
No Yes
ORDER BY Exists 6 ORDER BY
No Set Cursor
FIGURE 1-1 Logical query processing ow diagram
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Logical Query Processing
(1-J3) Add Outer Rows If OUTER JOIN is speci 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 lters the rows from VT1 based on the predicate that appears in the WHERE clause (<where_predicate>). 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 speci ed in the GROUP BY clause, generating VT3. Ultimately, there will be one result row per group. (4) HAVING This phase lters the groups from VT3 based on the predicate that appears in the HAVING clause (<having_predicate>). 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. This phase evaluates the expressions in the SELECT list,
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