Friday, 4 May 2012

T-SQL Logical Query Processing Phases

This section is lifted/quoting "Inside Microsoft SQL SERVER 2005: T-SQL Querying"
Logical Query Processing Phases
This section introduces the phases involved in the logical processing of a query.
I will first briefly 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.
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.

Listing 1-1 Logical query processing step numbers
(8) SELECT (9) DISTINCT (11) TOP
(1) FROM
(3) JOIN
(2) ON
(4) WHERE
(5) GROUP BY
(6) WITH {CUBE | ROLLUP}
(7) HAVING
(10) ORDER BY
The first noticeable aspect of SQL that is different than 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 first clause that is processed is the FROM clause,
while the SELECT clause, which appears first, 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 final step is returned to the caller. If a certain clause is not specified in a query, the
4 Inside Microsoft SQL Server 2005: T-SQL Querying
corresponding step is simply skipped. Following is a brief description of the different logical
steps applied in both SQL Server 2000 and SQL Server 2005. Later in the chapter, I will
discuss separately the steps that were added in SQL Server 2005.

Brief Description of Logical Query Processing Phases
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.

  1. FROM: A Cartesian product (cross join) is performed between the first two tables in the FROM clause, and as a result, virtual table VT1 is generated.
  2. ON: The ON filter is applied to VT1. Only rows for which the is TRUE are inserted to VT2.
  3. OUTER (join): If an OUTER JOIN is specified (as opposed to a CROSS JOIN or an INNER JOIN), rows from the preserved table or tables for which a match was not found are added to the rows from VT2 as outer rows, generating VT3. If more than two tables appear in the FROM clause, steps 1 through 3 are applied repeatedly between the result of the last join and the next table in the FROM clause until all tables are processed.
  4. WHERE: The WHERE filter is applied to VT3. Only rows for which the is TRUE are inserted to VT4.
  5. GROUP BY: The rows from VT4 are arranged in groups based on the column list specified in the GROUP BY clause. VT5 is generated.
  6. CUBE | ROLLUP: Supergroups (groups of groups) are added to the rows from VT5, generating VT6.
  7. HAVING: The HAVING filter is applied to VT6. Only groups for which the is TRUE are inserted to VT7.
  8. SELECT: The SELECT list is processed, generating VT8.
  9. DISTINCT: Duplicate rows are removed from VT8. VT9 is generated.
  10. ORDER BY: The rows from VT9 are sorted according to the column list specified in the ORDER BY clause. A cursor is generated (VC10).
  11. TOP: The specified number or percentage of rows is selected from the beginning of
    VC10. Table VT11 is generated and returned to the caller.

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