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I have a delete statement that is using a bad plan when run inside a stored procedure, but is choosing a much better plan when run ad-hoc.

I have rebuilt all the indexes for the tables used by the query and dropped all of the caches. The optimizer still chooses the wrong plan for the stored procedure.

I'd like to know why the optimizer is using a different execution plan for the stored procedure versus the ad-hoc SQL.

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2 Answers 2

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Usual suspects:

  1. constants in adhoc, parameters in code.
    The optimizer can then choose the best plan for the constants.
    Change the constants = change the plan. A parameterized plan is reusable.

  2. mismatch of data types in code.
    will introduce implicit conversions because of datatype precedence.
    eg varchar column compared to nvarchar parameter.

  3. parameter sniffing.
    use parameter masking or OPTIMISE FOR UNKNOWN

To test: run stored proc, run sp_updatestats, run again. This will invalidate cached plans which is better than clearing the entire plan cache.

You can disable sniffing several ways. The main 3 are

  • RECOMPILE. This is silly in my opinion.
  • OPTIMIZE FOR UNKNOWN
  • Parameter masking

Parameter masking looks like this:

DECLARE @MaskedParam varchar(10)
SELECT @MaskedParam = @SignaureParam

SELECT...WHERE column = @MaskedParam

Masking and the OPTIMIZE hint have the same effect (maybe for different reasons). That is, the optimizer has to use statistics and data distribution evaluate the parameters on their own merits ? , rather than what they were last call. The optimizer can recompile or not. SQL Server 2005 added statement level recompilation so there was less impact.

Now, why a plan with "sniffed" parameters is "sticky" compared to masked/"unknown" parameters, I'm not sure.

I've used parameter masking since SQL Server 2000 for all but the simplest code. I've noted that it is liable to happen with more complex code. And at my old job I has some report procs that I could change the plan parameter defaults. I reckon the "cargo cult" approach was easier than a support call.

  • Parameter masking and OPTIMISE FOR UNKNOWN have the same effect as far as I can tell.
    The hint is cleaner than masking but was added with SQL Server 2008.

  • Parameter sniffing happens at compile time.
    WITH RECOMPILE generates a new plan each execution. This means a poor choice of defaults will influence the plan. At my last job, I could demonstrate this easily with some report code: changing parameter defaults altered the plan regardless of supplied parameters.

  • This MS Connect article is interesting: Suboptimal index usage within stored procedure (mentioned in one of the SO answers below)

  • Bob Beauchemin mentions it too

Outstanding issues

  • Does sniffing still apply with WITH RECOMPILE? That is, if the optimizer knows to discard the plan does it aim for re-use?

  • Why are sniffed plans "sticky"?

Links from SO:

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  • 1
    Re: point 3. You can also run the statement with OPTION (RECOMPILE) or the whole proc WITH RECOMPILE to force SQL Server to disregard existing plans. Oct 4, 2011 at 16:53
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Don't forget that the ANSI settings that you have setup for the connection plan a role in execution plan selection. When the app calls the stored procedure it probably has different ANSI settings than your SSMS connection.

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