create virtual index - just for the plan

Question

I am in the process of optimizing a long running T-SQL query. Somehow, I remember there is a feature in SQL Server that allows checking how a plan would be if an index would exist. Something like CREATE VIRTUAL INDEX and then checking the plan. But I do not find how to achieve this.

Is there a feature like this in SQL Server 2008R2?

Answer 1

Yes you can, they are called hypothetical indexes, not virtual indexes. They are normally created by the database tuning advisor and are ignored by the query optimiser unless you run the query in autopilot mode.

So, to do this you need to do the following:

Create your hypothetical index on your table:

CREATE NONCLUSTERED INDEX IX_Hypothetical ON dbo.tableName (columnName) 
WITH STATISTICS_ONLY = -1

This creates an entry in the sys.indexes table with the is_hypothetical flag set to 1 (i.e. the index doesn't actually exist, just the statistics)

You then need to find out some information about the index to give to the autopilot, you need the database id, the object id of the table and the id number of the index, which you can get with the following query:

SELECT dbid = DB_ID(),
       objectid = 
object_id,
       indid = index_id
  FROM sys.indexes
 WHERE 
object_id = 
OBJECT_ID('dbo.tableName')
   AND is_hypothetical = 1

In my case, dbid = 7, objectid = 1237579447 and indid = 4

You can then run your query in autopilot mode to get an execution plan that could be generated if the hypothetical index you created actually existed on your table:

DBCC AUTOPILOT(0, 7, 1237579447, 4)
GO
SET AUTOPILOT ON
GO
SELECT * FROM dbo.tableName
WHERE columnName = 8
GO
SET AUTOPILOT OFF

The 0 is the typeid of the object, 0 is nonclustered index and I believe 6 is clustered index, you can run DBCC AUTOPILOT multiple times with different indexes if you have created more than one before running SET AUTOPILOT ON so that you can get the optimiser to evaluate all of them.

Also bear in mind that this is completely undocumented by Microsoft so not recommended for use outside of a dev machine and could change between versions with no warning, so don't rely too heavily on it working like this.