Parameter Sniffing Explained

Bert Wagner looks at how parameter sniffing works, why it’s (mostly) a good thing, and how to gently explain to the server when it’s a bad thing:

All subsequent executions of that same query will go to the query cache to reuse that same initial query plan — this saves SQL Server time from having to regenerate a new query plan.

Note: A query with different values passed as parameters still counts as the “same query” in the eyes of SQL Server.

In the case of the examples above, the first time the query was executed was with the parameter for “Costa Rica”. Remember when I said this dataset was heavily skewed? Let’s look at some counts:

Check it out for a clear depiction of the topic.  One solution that Bert doesn’t have but I will use sometimes is to create local variables in a procedure and set their values equal to input parameters.  That way, the optimizer doesn’t have an assumption as to the value of the local variable.  But there are several ways to get around this when it’s an issue.

Related Posts

Table Variables And Parallelism

Erik Darling shows your brain on table variables: Inserts and other modifications to table variables can’t be parallelized. This is a product limitation, and the XML warns us about it. The select could go parallel if the cardinality estimate were more accurate. This could potentially be addressed with a recompile hint, or with Trace Flag […]

Read More

Non-Blocking Aggregations

Daniel Hutmacher tilts at windmills: It’s not entirely uncommon to want to group by a computed expression in an aggregation query. The trouble is, whenever you group by a computed expression, SQL Server considers the ordering of the data to be lost, and this will turn your buttery-smooth Stream Aggregate operation into a Hash Match […]

Read More


August 2017
« Jul Sep »