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Microsoft® SQL Server 2008 R2 Unleashed
book

Microsoft® SQL Server 2008 R2 Unleashed

by Ray Rankins, Paul Bertucci, Chris Gallelli, Alex T. Silverstein
September 2010
Intermediate to advanced content levelIntermediate to advanced
1704 pages
111h 8m
English
Sams
Content preview from Microsoft® SQL Server 2008 R2 Unleashed

Estimating Rows Using Index Statistics

How does the Query Optimizer use the index statistics to estimate the number of rows that match the SARGs in a query?

SQL Server uses the histogram information when searching for a known value being compared to the leading column of the index key column, especially when the search spans a range or when there are duplicate values in the key. Consider this query on the sales table in the bigpubs2008 database:

image

Because there are duplicates of title_id in the table, SQL Server uses the histogram on title_id (refer to Listing 34.5) to estimate the number of matching rows. For the value of BI3976, it would look ...

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Publisher Resources

ISBN: 9780768696585Purchase book