BUG: Wrong Join Plan Selected That Causes Excessive Reads

• Microsoft SQL Server, version 6.5 Service Pack 2

SYMPTOMS

The SQL Server optimizer sometimes does not choose the optimal join order. It may reformat the larger table involved in a join followed by excessive logical and physical reads on the worktable, which can introduce serious performance degradation.

CAUSE

The method used to calculate the cost of each possible join plan has been altered in SQL Server 6.5 Service Pack 2 to provide more accurate and thorough estimates. Incidentally, it introduced a very small window where it may calculate the cost incorrectly. In such cases, the best join plan, which was chosen by both the release build of SQL Server 6.5 and by Service Pack 1, is considered to be very costly.

WORKAROUND

The combination of SET FORCEPLAN ON, optimizer hint on index selection, and possibly trace flag 336 may force the optimizer to use the right join plan.

STATUS

Microsoft has confirmed this to be a problem in Microsoft SQL Server version 6.5 Service Pack 2. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.