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Heap Pointer to first IAM
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IAM Allocation Order Scan
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FIGURE 4-20 Table scan
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An instruction of the optimizer in the execution plan to perform a table scan can be carried out by the storage engine only in one way using an allocation order scan. That is, SQL Server uses the table s IAM pages to scan the extents belonging to the table by their le order. As long as there s no le system fragmentation, the activity is done as a sequential activity in the disk drives. The number of logical reads should be similar to the number of pages the table consumes (around 25,000 in our case). Note that in such scans SQL Server typically uses a very ef cient read-ahead strategy that can read the data in larger chunks than 8 KB. When I ran this query on my system, I got the following performance measures from STATISTICS IO, STATISTICS TIME:
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Logical reads 24391 Physical reads 3 Read-ahead reads 24368 CPU time 951 ms Elapsed time 23935 ms Estimated subtree cost 19.1699
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Of course, the run times I got are not an indication of the run times you would get in an average production system. But I wanted to show them for illustration and comparison purposes. If the table has a clustered index, the access method that will be applied will be an unordered clustered index scan (that is, a Clustered Index Scan operator, with the property Ordered: False). Figure 4-21 shows the execution plan that the optimizer will produce for this query. Notice that the Ordered property of the Clustered Index Scan operator indicates False. Figure 4-22 shows an illustration of the two ways that the storage engine can carry out this access method.
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FIGURE 4-21 Unordered clustered index scan (execution plan)
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Inside Microsoft SQL Server 2008: T-SQL Querying
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Clustered Index
Pointer to first leaf page Index Order Scan
Clustered Index
Pointer to first IAM
Allocation Order Scan
FIGURE 4-22 Unordered clustered index scan
The fact that the Ordered property of the Clustered Index Scan operator indicates False means that as far as the relational engine is concerned, the data does not need to be returned from the operator ordered. This doesn t mean that it is a problem if it is returned ordered; instead, it means that any order would be ne. This leaves the storage engine with some maneuvering space in the sense that it is free to choose between two types of scans: an index order scan (scan of the leaf of the index following the linked list) and an allocation order scan (scan based on IAM pages). The factors that the storage engine takes into consideration when choosing which type of scan to employ include performance and data consistency. I ll provide more details about the storage engine s decision-making process after I describe ordered index scans (Clustered Index Scan and Index Scan operators with the property Ordered: True). Here are the performance measures I got for this query:
Logical reads 25081 Physical reads 5 Read-ahead reads 25073 CPU time 889 ms
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Query Tuning
Elapsed time 24025 ms Estimated subtree cost 19.6218
Unordered Covering Nonclustered Index Scan
An unordered covering nonclustered index scan is similar in concept to an unordered clustered index scan. The concept of a covering index means that a nonclustered index contains all columns speci ed in a query. In other words, a covering index is not an index with special properties; rather, it becomes a covering index with respect to a particular query. SQL Server can nd all the data it needs to satisfy the query by accessing solely the index data, without the need to access the full data rows. Other than that, the access method is the same as an unordered clustered index scan, only, obviously, the leaf level of the covering nonclustered index contains fewer pages than the leaf of the clustered index because the row size is smaller and more rows t in each page. I explained earlier how to calculate the number of pages in the leaf level of an index (clustered or nonclustered). As an example for this access method, the following query requests all orderid values from the Orders table:
SELECT orderid FROM dbo.Orders;
Our Orders table has a nonclustered index on the orderid column (PK_Orders), meaning that all the table s order IDs reside in the index s leaf level. The index covers our query. Figure 4-23 shows the graphical execution plan you would get for this query, and Figure 4-24 illustrates the two ways in which the storage engine can process it.
FIGURE 4-23 Unordered covering nonclustered index scan (execution plan)
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