It is far easier to work out what is wrong with a slow-running SQL query than one that shows erratic performance. DBAs will groan when a process occasionally performs terribly without apparent reason. There is always a good chance that this is a symptom of the painful problem of parameter sniffing. It can afflict database applications badly enough that SQL Server 2016 introduces an option to disable sniffing at the database level.
Let’s say you have a stored procedure or parameterized query that allows you the flexibility of providing a specific ProductID (PID) value or setting it to null, in which case you want data for all products. When the SQL Server optimizer creates or recompiles an execution plan for this query, it will “sniff” the input parameter value, let’s say we provided a value of 10. It uses column statistics to estimate how many rows might be returned for a PID of 10 (40 rows, say), selects an appropriate plan, passes it on for execution, and places it in the plan cache.
That last part is what occasionally causes trouble. If the optimizer sees the same query text again, just with different parameter values, it checks the plan cache and reuses the existing plan, if it’s in there. This is fine but if subsequent executions set the PID to NULL, and the Product table is large, then we’re now using a plan devised to return 40 rows for a query execution that actually returns thousands. That is likely to end badly. There are other common causes of this problem too, such as filtering on columns with skewed data distributions, or filtering on a date range and ending up with a cached plan for a very atypical range.
There’s no guaranteed way of avoiding this, but you can, during development, detect a procedure that is likely to cause problems. You first clear the cache to prevent the optimizer just grabbing the cached plan, and then capture the estimated execution plan for several different executions, using different parameter values. Estimated plans don’t get cached, and you’ll see the optimizer’s plan for each individual execution. If you’re seeing different estimated plans for some parameter values, that’s a warning sign. If it’s a flexible stored procedures that performs a range of duties, you should consider refactoring it. In any event, it’s an incentive to check how bad the performance gets if certain executions end up using the wrong plan, because that’s what is in the plan cache at that time.
This sort of testing can become tedious but you can automate it by using a test harness to capture the estimated plans for a wide range of parameter values. If you see different plans then depending on the frequency with which each one occurs, you have firmer evidence on which to base a decision to refactor the code, use an OPTIMIZE FOR hint, or even recompile the stored procedure on every execution. Of course, even this technique becomes impractical if the procedure accepts many parameters: you might end up needing to test millions of different possible executions.
I’m interested to hear how badly you’ve been stung by parameter sniffing problems, and any techniques you’ve used to gauge the extent of the likely problem, during testing.