SQL Server – When Does sp_executesql Refresh the Query Plan?

execution-plansql serverstored-procedures

You'll have to forgive my naivety as I'm not a DBA but my understanding is that over time the statistics of a database change and a stored procedure must be recompiled to keep the query plan up to date with the latest statistics.

Assuming I have a stored procedure in my database that is recompiled against the latest statistics at some regular interval, what are the implications of in-lining the stored procedure in code and wrapping it in an sp_executesql statement? Do I lose the refreshing of the query plan that used to happen as part of the recompilation of the procedure?

If there's anything else (other than permissions) that I need to consider before I make this change then I'd appreciate your insights.

I read this on MSDN:

The ability of the SQL Server query optimizer to match the new
Transact-SQL string with an existing execution plan is hampered by the
constantly changing parameter values in the text of the string,
especially in complex Transact-SQL statements.

So assuming the stored procedure I'm attempting to in-line and wrap in sp_executesql does indeed contain some parameters, is this saying that although my execution plan is cached, I'm making it more difficult for SQL Server to find and reuse it?

Best Answer

The line from MSDN is talking about using EXEC(), like this:

SET @sql = 'SELECT foo FROM dbo.bar WHERE x = ''' + @x + ''';';
EXEC(@sql);

In my testing, modern versions of SQL Server are still able to reuse a plan like this, but there may be other variables (such as version, or for example if you add conditional WHERE clauses based on the presence of certain parameters - in which case that will generate a different plan).

If you use sp_executesql then the parameter values can still cause parameter sniffing issues (just like with normal SQL), but this has nothing to do with whether SQL Server can re-use the plan. This plan will get used over and over again, just as if you hadn't used sp_executesql at all, unless variables that would cause a direct query to get recompiled, in which case this one will get recompiled too (essentially, SQL Server doesn't store anything with the plan that says "this was executed from sp_executesql, but this one wasn't):

SET @sql = N'SELECT foo FROM dbo.bar WHERE x = @x;';
EXEC sys.sp_executesql @sql, N'@x varchar(32)', @x;

As a bonus, this has built-in protection against dynamic SQL, and avoids you having to worry about doubling up single quotes due to string delimiters. I blogged about some of this here and please read up on SQL injection here and here.

If you are having issues with plan re-use and/or parameter sniffing, some things you should look into are OPTION (RECOMPILE), OPTIMIZE FOR, optimize for ad hoc workloads and simple/forced parameterization. I addressed a few similar questions in response to a recent webcast here, it may be worth a skim:

The gist is: don't be afraid to use sp_executesql, but only use it when you need it, and only spend energy over-optimizing it when you have an actual performance issue. The example above is a terrible one because there's no reason to use dynamic SQL here - I've written this answer assuming you have a legitimate use case.

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