Question

I have a sql server 2008R2 with a database that have table with thousands of rows. When i start the server execute fast this query:

exec sp_executesql N'SELECT *, [t0].[distance] AS [Meters], @p4 AS [ToWhat]
FROM [dbo].[getEventsByRange](@p0, @p1, @p2, @p3) AS [t0]
LEFT JOIN [dbo].[Event] AS [t1] ON [t0].[idEvent] = [t1].[id]
ORDER BY [t0].[distance]',N'@p0 var

where getEventsByRange is this function:

SELECT idEvent, geography::STGeomFromText('POINT(' + @userLongitude + ' ' +       @userLatitude + ')', 4326).STDistance([Coordinates]) as distance
FROM EventCoords
INNER JOIN Event ON EventCoords.idEvent = Event.id
WHERE DAY(start_time)=DAY(@timeMax) AND MONTH(start_time)=MONTH(@timeMax)AND
YEAR(start_time)=YEAR(@timeMax) and 
geography::STGeomFromText('POINT(' + @userLongitude + ' ' + @userLatitude + ')', 4326).STDistance([Coordinates]) < @maxDistance

After some minutes this query become very slow to execute. Where is the problem? Why when the server haven’t query cached go fast?

Answer 1

Could it be that Parameter Sniffing is causing the reuse of a suboptimal cached execution plan ?

To see if this is the case and OPTION (RECOMPILE) to the end of your query:

ORDER BY [t0].[distance]
OPTION (RECOMPILE)

If the queries stay performant (and thus the problem is parameter sniffing) we can make some better recommendations