upc internet csomagok FIGURE 8-7 Plan for query using INNER JOIN instead of subquery in Visual Basic .NET

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FIGURE 8-7 Plan for query using INNER JOIN instead of subquery
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FIGURE 8-8 Plan for query using DISTINCT and INNER JOIN instead of subquery
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The Query Optimizer
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FIGURE 8-9 Plan for query using subquery
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In this last query plan, the matching rows from the Customers table are kept and directly returned to the user through the Left Semi-Join operator. Note The Left and Right Semi-Join have to do with which child s rows are preserved in the
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operation. Unfortunately for anyone confused as to the meaning of these operators, the plan representation in SQL Server Management Studio and in previous tools is transposed. The left child is the top child and the right child is the bottom child in the transposed form.
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Apply
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CROSS APPLY and OUTER APPLY were added to SQL Server 2005, and they represent a special kind of subquery where a value from the left input is passed as a parameter to the right child. This is sometimes called a correlated nested loops join, and it represents passing a parameter to a subquery. The most common application for this feature is to do an index lookup join, as seen in Listing 8-2 and Figure 8-10.
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LISTING 8-2 Example of APPLY Query
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CREATE TABLE idx1(col1 INT PRIMARY KEY, col2 INT); CREATE TABLE idx2(col1 INT PRIMARY KEY, col2 INT); GO SELECT * FROM idx1 CROSS APPLY ( SELECT * FROM idx2 WHERE idx1.col1=idx2.col1 ) AS a;
FIGURE 8-10 APPLY query plan
Microsoft SQL Server 2008 Internals
This query is logically equivalent to an INNER JOIN, and Figure 8-11 demonstrates that the resulting query plan is identical in SQL Server 2008.
SELECT * FROM idx1 INNER JOIN idx2 ON idx1.col1=idx2.col1;
FIGURE 8-11 INNER JOIN query generates a nested loop and seek plan
In both cases, a value from the outer table is referenced as an argument to the seek on the inner table. Notice that a standard inner join is also able to generate a seek, which means that the Query Optimizer considers turning a JOIN into an APPLY as part of the optimization process. Although the example I have done here is so simple that you would not need to write the query in the way that I did, there are more complex scenarios where this syntax is useful. First, there is a common pattern for dynamic management views (DMVs, including an example in the section entitled Plan Hinting, later in this chapter) where a value is passed to a management function using a cross apply. Second, there may be rare, very complex cases when the Query Optimizer s rule engine cannot rewrite a simple inner join to get an index seek. In those cases, rewriting the query to use CROSS APPLY is useful to pass a parameter down past an opaque operator manually. The semantics of a query can change as a result of a rewrite like this, so be very sure that you understand the semantics of your query when considering a rewrite like this. The Apply operator is almost like a function call in a procedural language. For each row from the outer (left) side, some logic on the inner (right) side is evaluated and zero or more rows are returned for that invocation of the right sub-tree. The Query Optimizer can sometimes remove the correlation and convert an Apply into a more general join, and in those cases other joins can sometimes be reordered to explore different plan choices.
Spools
SQL Server has a number of different, specialized spools. Each one is highly tuned for some scenario. Conceptually, they all do the same thing they read all the rows from the input, store it in memory or spill it to disk, and then allow operators to read the rows from this cache. Spools exist to make a copy of the rows, and this can be important for transactional consistency in some update plans and to improve performance by caching a complex subexpression to be used multiple times in a query.
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