Author: Kevin Feasel

Grant Fritchey wonders, if you force a plan using Query Store but the plan ages out of cache, do you still use the forced plan?

To start with, a small stored procedure that I use all the time for bad parameter sniffing demos that reliably gets different plans with different values due to statistics skew:

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CREATE PROC dbo.spAddressByCity @City NVARCHAR(30) AS SELECT  a.AddressID,         a.AddressLine1,         a.AddressLine2,         a.City,         sp.Name AS StateProvinceName,         a.PostalCode FROM    Person.Address AS a JOIN    Person.StateProvince AS sp         ON a.StateProvinceID = sp.StateProvinceID WHERE   a.City = @City;

If this procedure is called for a value of ‘London’ it gets a plan with a Merge Join. For most other value it gets a plan with a Loops Join. Here’s an example of the ‘London’ plan:

It’s a good question with a good answer.

Erin Stellato talks about default query store settings:

For each database where I enable Query Store, I’d consider the workload and then look at the settings.  I tend to think that the default value of 100MB for MAX_STORAGE_SIZE_MB is really low, and I would be inclined to bump up STALE_QUERY_THRESHOLD_DAYS from 30 to something a bit higher.  I’d also probably drop  DATA_FLUSH_INTERVAL_SECONDS to something lower than 900 seconds (15 minutes) if my storage can support it.  This setting determines how often Query Store data is flushed to disk.  If it’s every 15 minutes, then I could potentially lose 15 minutes of Query Store data if my server happened to crash before it could be written to disk.  I’d also think about changing INTERVAL_LENGTH_MINUTES to a value smaller than 60 if I wanted to aggregate my query data over a smaller amount of time.  Sometimes interesting events happen within a 60 minute time frame, and they get can lost when data is aggregated across that window.  However, aggregating more frequently means I’m adding processing overhead to the system – there’s a trade-off there to figure out.

In our environment at least, 100 MB of query store data would last, oh, a couple hours?  Definitely tweak your settings and keep an eye on them early on.