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
select * from Person where LastName = 'Alexander'
LastName values in the table are not unique, SQL Server uses the histogram on
LastName (refer to Listing 32.2) to estimate the number of matching rows. For the value of
'Alexander', it would look at ...