Parameter Sniffing (or Spoofing) in SQL Server

Question

A while ago I had a query that I ran quite a lot for one of my users. It was still being evolved and tweaked but eventually it stablised and ran quite quickly, so we created a stored procedure from it.

So far, so normal.

The stored procedure, though, was dog slow. No material difference between the query and the proc, but the speed change was massive.

[Background, we're running SQL Server 2005.]

A friendly local DBA (who no longer works here) took one look at the stored procedure and said "parameter spoofing!" (Edit: although it seems that it is possibly also known as 'parameter sniffing', which might explain the paucity of Google hits when I tried to search it out.)

We abstracted some of the stored procedure to a second one, wrapped the call to this new inner proc into the pre-existing outer one, called the outer one and, hey presto, it was as quick as the original query.

So, what gives? Can someone explain parameter spoofing?

Bonus credit for

• highlighting how to avoid it
• suggesting how to recognise possible cause
• discuss alternative strategies, e.g. stats, indices, keys, for mitigating the situation

Answer 1

Yes, I think you mean parameter sniffing, which is a technique the SQL Server optimizer uses to try to figure out parameter values/ranges so it can choose the best execution plan for your query. In some instances SQL Server does a poor job at parameter sniffing & doesn't pick the best execution plan for the query.

I believe this blog article http://blogs.msdn.com/queryoptteam/archive/2006/03/31/565991.aspx has a good explanation.

It seems that the DBA in your example chose option #4 to move the query to another sproc to a separate procedural context.

You could have also used the with recompile on the original sproc or used the optimize for option on the parameter.

Answer 2

FYI - you need to be aware of something else when you're working with SQL 2005 and stored procs with parameters.

SQL Server will compile the stored proc's execution plan with the first parameter that's used. So if you run this:

usp_QueryMyDataByState "Rhode Island"

The execution plan will work best with a small state's data. But if someone turns around and runs:

usp_QueryMyDataByState "Texas"

The execution plan designed for Rhode-Island-sized data may not be as efficient with Texas-sized data. This can produce surprising results when the server is restarted, because the newly generated execution plan will be targeted at whatever parameter is used first - not necessarily the best one. The plan won't be recompiled until there's a big reason to do it, like if statistics are rebuilt.

This is where query plans come in, and SQL Server 2008 offers a lot of new features that help DBAs pin a particular query plan in place long-term no matter what parameters get called first.

My concern is that when you rebuilt your stored proc, you forced the execution plan to recompile. You called it with your favorite parameter, and then of course it was fast - but the problem may not have been the stored proc. It might have been that the stored proc was recompiled at some point with an unusual set of parameters and thus, an inefficient query plan. You might not have fixed anything, and you might face the same problem the next time the server restarts or the query plan gets recompiled.

Answer 3

Parameter sniffing is a technique, Sql Server uses to optimze the query execution plan for a stored procedure. When you first call the SP, Sql Server looks at the given parameter values of your call and decides which indices to use based on the parameter values.

So when the first call contains not very typical parameters, Sql Server might select and store a suboptimal execution plan in regard to the following calls of the sp.

You can work around this by either

• using WITH RECOMPILE
• copying the parameter values to local variables inside the sp and using the locals in your queries.

I even heard that its better to not use procs at all but send your queries directly to the server. I recently came across the same problem where i have no real solution yet. For some queries the copy to local vars helps getting back to the right execution plan, for some queries performance degrades with local vars.

I still have to do more research on how Sql Server caches and reusese (suboptimal) execution plans.