Microsoft® SQL Server 2012 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 Person table in the AdventureWorks2012 database:

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select * from Person    where LastName = 'Alexander'

Because there are duplicates of LastName in the table, SQL Server uses the histogram on LastName (refer to Listing 30.5) to estimate the number of matching rows. For the value of 'Alexander', it would look at the ...