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tabLe 6-1 Theoretical Execution Order Excluding the UNION Clause
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FROM, JOIN, APPLY, and ON WHERE GROUP BY and aggregate functions (such as SUM, AVG, and so on) that are included in the query
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The join is executed and the first query filter (the ON clause) is applied. The second query filter is applied. Grouping and aggregation calculations are performed.
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Techniques to Improve Query Performance
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HAVING SELECT ORDER BY TOP
The third query filter (filtering of the results of aggregate functions) is applied. Columns that should be returned by the query are selected. Results are sorted. The fourth (and last) query filter is applied; this causes the query to return only the first X rows from the results thus far. The tabular result returned by the SELECT statement is converted to Extensible Markup Language (XML).
FOR XML
Queries that use the UNION clause use the theoretical execution order shown in Table 6-2.
tabLe 6-2 Theoretical Execution Order Including the UNION Clause
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1. 2. 3.
FROM, JOIN, APPLY, and ON WHERE GROUP BY and aggregate functions (such as SUM, AVG, etc.) that are included in the query HAVING TOP
The join is executed and the first query filter (the ON clause) is applied. The second query filter is applied. Grouping and aggregation calculations are performed. The third query filter (filtering of the results of aggregate functions) is applied. The fourth (and last) query filter is applied; this causes the query to return only the first X rows from the results thus far. (Note that in this case, the TOP clause is executed before the ORDER BY clause.) The results of each SELECT statement included in the query are concatenated; columns that should be returned by the query are selected. The results are sorted. The tabular result returned by the SELECT with UNION statement is converted to XML.
4. 5.
UNION and SELECT
7. 8.
ORDER BY FOR XML
Lesson 1: Tuning Queries
The cause of the difference in the execution order is the introduction of the TOP clause (in SQL Server 7.0), which is not part of the ANSI/ISO SQL standard. The standard-compliant behavior of the UNION clause allows only one ORDER BY clause, which must be placed in the last SELECT statement of the query and must sort the entire query result. This means the TOP clause in any but the final SELECT statement of a UNION query returns the top X items before they are sorted. This may sound like a minor point, but it is important to be aware of. For example, compare the result of two queries, both of which are intended to return the two most expensive red products and the two most expensive black products. The first query, shown here, produces an incorrect result:
USE AdventureWorks;
SELECT TOP(2) ProductID, Name, Color, ListPrice FROM Production.Product WHERE Color = 'Black' UNION SELECT TOP(2) ProductID, Name, Color, ListPrice FROM Production.Product WHERE Color = 'Red' ORDER BY ListPrice DESC;
Here is the (incorrect) result of this first query:
ProductID Name Color ListPrice ----------- ------------------------- ----- ---------706 707 317 318 HL Road Frame - Red, 58 Sport-100 Helmet, Red LL Crankarm ML Crankarm Red Red 1431,50 34,99
Black 0,00 Black 0,00
The second query, shown here, generates the intended result:
USE AdventureWorks;
WITH a AS ( SELECT TOP(2) ProductID, Name, Color, ListPrice FROM Production.Product WHERE Color = 'Black' ORDER BY ListPrice DESC ), b AS ( SELECT TOP(2) ProductID, Name, Color, ListPrice FROM Production.Product WHERE Color = 'Red' ORDER BY ListPrice DESC ) SELECT * FROM a UNION ALL SELECT * FROM b;
Techniques to Improve Query Performance
Here are the (correct) results of this second query:
ProductID Name Color ListPrice ----------- ------------------------- ----- ---------775 776 749 750 Mountain-100 Black, 38 Mountain-100 Black, 42 Road-150 Red, 62 Road-150 Red, 44 Black 3374,99 Black 3374,99 Red Red 3578,27 3578,27
As you can see, the first query does not return the correct values because the ORDER BY clause is executed after the TOP clause.
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