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Bug #: SHILOH_BUGS (470434)
A query may run slower against Microsoft SQL Server 2000 post-SP3 hotfix build 8.00.0776 or later when compared to queries that run against SQL Server 2000 before hotfix build 8.00.0776, if the following conditions are true:
The query involves a left outer join of two tables. In the following example, table 1 is the left table and table 2 is the right table.
An IS NULL filter condition exists on a column for the right table (table 2).
The joining columns have different data types. One column is of type varchar/char, and the other column is of type nvarchar/nchar.
Very few of the query result rows satisfy the ON condition of the outer join.
For example, a query that contain the following join and filter condition may run slower:
t1 left outer join t2 on t1.c1 = t2.c1 where t2.c1 is null --t1.c1 is of type nvarchar and t2.c1 is of type varchar
This problem occurs because the SQL Server optimizer underestimates the number of rows that satisfies the IS NULL filter condition after the join.
To work around this problem, make sure that the joining columns are of the same data type.
Service pack information
To resolve this problem, obtain the latest service pack for Microsoft SQL Server 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
290211 How to obtain the latest SQL Server 2000 service pack
The English version of this hotfix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
Note Because of file dependencies, the most recent hotfix or feature that contains these files may also contain additional files.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section of this article.
This problem was first corrected in Microsoft SQL Server 2000 Service Pack 4.
After you install the hotfix that is described in this article, you must make sure that trace flag 9209 is turned on when you start SQL Server. For this fix to take effect, you must have trace flag 9209 turned on when you start SQL Server. To do so, follow these steps:
Start SQL Server Enterprise Manager.
Right-click the instance of SQL Server, and then click Properties.
Click the General tab, and then click Startup Parameters.
In the Startup Parameters dialog box, type -T9209 in Parameter box, click Add, and then click OK.
In the SQL Server Properties dialog box, click OK.
Restart SQL Server.
Data type conversions (in this case, when an nvarchar/nchar column is compared to a varchar/char column) on a column in a query may have an adverse effect on the query's performance, and it is a good idea to avoid doing so if you can. If you use data type conversions, SQL Server may not be able to accurately produce cardinality estimates. SQL Server may also be restricted from using appropriate indexes. For additional information about data type conversions, click the following article number to view the article in the Microsoft Knowledge Base:
271566 PRB: SQL Server comparisons between columns and constants with different data types