Databases Reference
In-Depth Information
Finally, drop the statistics object you have just created, by running the
following statement:
DROP
STATISTICS
Person
.
Address
.
california
Listing 3-45.
Cardinality Estimation Errors
Cardinality estimation errors can lead to the Query Optimizer making poor choices
as to how best to execute a query and, therefore, to badly performing execution plans.
Fortunately, you can easily check if you have cardinality estimation errors by comparing
the estimated against the actual number of rows, as shown on graphical or XML
execution plans, or by using the
SET
STATISTICS
PROFILE
statement. In the next query,
I'll show you how to use the
SET
STATISTICS
PROFILE
statement with one of our
previous examples where SQL Server is making a blind guess regarding the selectivity of
certain columns:
SET
STATISTICS
PROFILE
ON
GO
SELECT
*
FROM
Sales
.
SalesOrderDetail
WHERE
OrderQty
*
UnitPrice
>
10000
GO
SET
STATISTICS
PROFILE
OFF
GO
Listing 3-46.
