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The main optimization path in SQL Server is a very powerful cost-based model of the execution time of a query. As databases and queries over those databases have become larger and more complex, this model has allowed SQL Server to solve bigger and bigger business problems. The xed startup cost for running this model can be expensive for applications that are not trying to perform complex operations. Making a single path that spans from the smallest to the largest queries can be challenging, as the requirements and speci cations are vastly different. To be able to satisfy small query applications well, a fast path was added to SQL Server to identify queries where cost-based optimization was not needed. Generally, this code identi es cases where a query does not have any cost-based choices to make. This means
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Microsoft SQL Server 2008 Internals
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that there is only one plan to execute or there is an obvious best plan that can be identi ed. In these cases, the Query Optimizer directly generates the best plan and returns it to the system to be executed. For example, the query SELECT col1 FROM Table1 for a table without any indexes has a straightforward best plan choice read the rows from the base table heap and return them to the user, as seen in Figure 8-15.
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CREATE TABLE Table1 (col1 INT, col2 INT); SELECT col1 FROM Table1;
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FIGURE 8-15 Trivial plan example table scan
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The SQL Server query processor actually takes this concept one step further. When simple queries are compiled and optimized, the query processor attempts to turn these queries into a parameterized query. If the plan is determined to be trivial, the parameterized query is turned into an executable plan. Then, future queries that have the same shape except for constants in well-known locations in the query just run the existing compiled query and avoid going through the Query Optimizer at all. This speeds up applications with small queries on SQL Server signi cantly.
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SELECT col1 FROM Table1 WHERE col2 = 5; SELECT col1 FROM Table1 WHERE col2 = 6;
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If you look at the text of these queries in the procedure cache in Listing 8-4, you see that there is actually only one query plan, and it is parameterized.
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LISTING 8-4 Automatically Parameterized Query Text
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SELECT text FROM sys.dm_exec_query_stats AS qs CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st WHERE st.text LIKE '%Table1%'; ---------------------------------------(@1 tinyint)SELECT [col1] FROM [Table1] WHERE [col2]=@1
If you examine the XML plan for this query plan, you see that there is an indication that this query was a trivial plan. The other choice is full, meaning that cost-based optimization was performed):
. . . <StmtSimple . . . StatementOptmLevel="TRIVIAL"> . . .
(XML plan output is verbose, so I have omitted most of it for space. The bold code shows the choice the Query Optimizer made.)
8
The Query Optimizer
Limitations
The trivial plan optimization was introduced in SQL Server 7.0. While each version of SQL Server has slightly different rules, all versions have queries that skip the trivial plan stage completely and only perform regular optimization activities. Using more complex features can disqualify a query from being considered trivial because they always have a cost-based plan choice or are too dif cult to identify as trivial or not. Examples of query features that cause a query not to be considered trivial include Distributed Query, Bulk Insert, XPath queries, queries with joins or subqueries, and queries with hints, some cursor queries, and queries over tables containing ltered indexes. SQL Server 2005 added another feature, called forced parameterization, to auto-parameterize queries more aggressively. This feature parameterizes all constants, ignoring cost-based considerations. This feature is most useful for an application where the SQL is generated (and you cannot make it generate parameterized queries) and the resulting query plans are almost always identical (or the plans perform similarly even if they differ). Speci cally, it is worth considering when the application queries cannot be changed by the DBA in charge of the server. The bene t of this feature is that it can reduce compilations, compilation time, and the number of plans in the procedure cache. All these things can improve system performance. On the other hand, this feature can reduce performance when different parameter values would cause different plans to be selected. These values are used in the Query Optimizer s cardinality and property framework to make decisions about how many rows will be returned from each possible plan choice, and forced parameterization blocks these optimizations. So if you think that your application would bene t from using forced parameterization, perform some experiments and see whether the application works better. 9, Plan Caching and Recompliation, goes into more detail on the various parameterization options.
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