Creating Plan Guides

Brent Ozar has a couple examples of creating plan guides to work around bad queries using OPTIMIZE FOR UNKNOWN:

You can see the actual execution plan here, and hover your mouse over various parts of it to see the estimated vs actual rows.

The OPTIMIZE FOR UNKNOWN hint tells SQL Server to use the density vector rather than column statistics, so it only estimates that 1,865 rows will come back – when in actuality, 3.3mm rows come back. In performance tuning, that’s what we call a “bad thing,” since SQL Server ends up doing around 10mm page reads due to that key lookup. It would have been much more efficient to just do a clustered index scan.

Creating plan guides can be ugly business, but sometimes they’re the best solution.

Related Posts

COUNT(*) Versus COUNT(1)

Lukas Eder takes on the myth that COUNT(*) differs from COUNT(1): Now that we know the theory behind these COUNT expressions, what’s the difference between COUNT(*) and COUNT(1). There is none, effectively. The 1 expression in COUNT(1) evaluates a constant expression for each row in the group, and it can be proven that this constant expression will never evaluate to NULL, so effectively, we’re running COUNT(*), […]

Read More

Workload Analysis with Query Store

Erin Stellato shows how you can mine the Query Store tables to learn more about your workload: The query text and plan are stored in two separate tables, sys.query_store_query_text and sys.query_store_plan, respectively.  The text is stored as a nvarchar(max) data type, and the plan is stored as varbinary(max).  This means that the data can be […]

Read More

Categories

November 2016
MTWTFSS
« Oct Dec »
 123456
78910111213
14151617181920
21222324252627
282930