I'm trying to increase the performance of a query and I'm out of ideas:

The query does this: gets a 'Process' with specific priorities on selection namely:

Picks up a process that has been waiting for over 15 minutes, AND has either no 'sleep until' value set, or the 'sleep until' is less than the time NOW AND ProcessState = 1.

Possible values for ProcessState are 1,2,3,4

The Query I'm trying to optimize is below: it runs very frequently:

SELECT TOP (1) [t0].[ProcessID],
FROM [dbo].[WF_Process] AS [t0]
WHERE ([t0].[ProcessStateDateLastModified] <= '2015-06-18 09:48:31.597') 
AND (([t0].[SleepUntil] IS NULL) OR ([t0].[SleepUntil] <= '2015-06-18 10:03:31.597')) 
AND ([t0].[ProcessState] = 1) 
ORDER BY [t0].[ProcessStateDateLastModified]

The only things that change are the two date time fields, which operate a sliding window from DateTime.Now(). First paramter DateTime.Now()- 15 minutes, second date time paramater is DateTime.Now().

Execution plan here: https://gist.github.com/anonymous/fd56ead25d726d247518 Or execution plan diagram here: execution plan diagram

The table looks like this:

CREATE TABLE [dbo].[WF_Process](
    [ProcessID] [bigint] IDENTITY(1,1) NOT NULL,
    [MaterialID] [bigint] NOT NULL,
    [Progress] [decimal](18, 2) NULL,
    [ProcessStatusDescription] [varchar](900) NOT NULL,
    [WorkflowProcessState] [int] NOT NULL,
    [WorkflowProcessSubState] [int] NOT NULL,
    [ProcessState] [int] NOT NULL,
    [ProcessStateDateLastModified] [datetime] NOT NULL,
    [DateCreated] [datetime] NOT NULL,
    [DateLastChecked] [datetime] NULL,
    [SleepUntil] [datetime] NULL,
    [LongRunningProcessID] [varchar](65) NULL,
    [ProcessID] ASC




ALTER TABLE [dbo].[WF_Process]  WITH CHECK ADD  CONSTRAINT [FK_Process_GB_Material] FOREIGN KEY([MaterialID])
REFERENCES [dbo].[GB_Material] ([MaterialID])

ALTER TABLE [dbo].[WF_Process] CHECK CONSTRAINT [FK_Process_GB_Material]

I have a combined index on all the columns which are within the query: ( and I have other indexes on the table too )

enter image description here

There are 187,851 rows in the database with a data space of 50MB. It has a total Index space of 483MB.

The Query IO Statistics look like this:

(1 row(s) affected)
Table 'WF_Process'. Scan count 1, logical reads 27566, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

The client statistics looks like this: enter image description here

The DB server is meaty. DL360 gen8 ( 32 GB ram, 30+ cores etc ).

This query is very heavily used, and so I'm trying to optimize it as much as I can, any help/information to speed up this query would be appreciated. Anything over 500 ms isn't really acceptable.


Did as Zohar said and I now have [ProcessStateDateLastModified] as Key index column, and the other two columns [SleepUntil] and [ProcessState] as included columns. This reduced the number of logical rads ( expected ), but did not reduce the overall timings of the query:

(1 row(s) affected)
Table 'WF_Process'. Scan count 1, logical reads 741, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

Execution plan: enter image description here

Client stats: enter image description here

  • The key lookup is the main cause of IO. Take another look at your index. Commented Jun 18, 2015 at 10:17
  • Hello, thanks for your reply, what do you mean by take another look? Commented Jun 18, 2015 at 10:26
  • Add the columns in the select list to the index as included columns. this will eliminate the need for key lookup. (the price is, of course, a larger size index) Commented Jun 18, 2015 at 10:34
  • Hmm, can you briefly explain to me the difference between Index key columns and Included columns and what performance implications this has? Commented Jun 18, 2015 at 10:44
  • I probably can, but MSDN does it better... Commented Jun 18, 2015 at 10:53

1 Answer 1


I would suggest to try an index like this:

CREATE NONCLUSTERED INDEX NCI_ProcessState_ProcessStateDateLastModified_SleepUntil 
ON [TABLE](ProcessState,ProcessStateDateLastModified,SleepUntil)

This way you can filter sharp on the state as all others will result in bigger result sets due to the <= filter.

You can check the index usage easily with this query:

SELECT obj.*,usage.*
FROM sys.dm_db_index_usage_stats AS usage
INNER JOIN sys.indexes as obj
        ON usage.object_id = obj.object_id
WHERE database_id = db_id()

Just filter it on your index and take a look a the search and update columns.

  • Hmm, but doesn't the low carnality make it an inefficient index? Commented Jun 18, 2015 at 11:30
  • Well if you use the ProcessState later your SQL Server needs to filter your query by date <= which will produce a huge amount of possible datapages. Even if you have just state 0 and 1, the possible pages will be halfed for the second part of the index. I've added additional info in the answer.
    – Ionic
    Commented Jun 18, 2015 at 11:34
  • Hello, thanks for this, that helped massively. Commented Jun 18, 2015 at 12:05
  • Nice to read. :-)
    – Ionic
    Commented Jun 18, 2015 at 12:33

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