Sql-server – Warning for missing statistics in execution plan

Would it be a potential bottle neck on performance if a statistic had already been created automatically for the same column(s) by SQL Sever and my script creates a duplicate statistic?

Surely you can write your script such that it detects such a duplicate statistic, instead of just blindly creating another one (possibly multiple times)? We do this for tables, views, procedures, functions, indexes... why not for statistics too? If you have the ability to create a statistic you have the ability to check the metadata for one that's already there (and maybe just needs to be updated).

I haven't done any performance tests, and I'm not sure if auto-stats (sync or async) will kick off multiple times or just pick one of them, but certainly nothing good can come of it. In the one case you're going to spend twice (or more!) the amount of time updating redundant statistics; if they don't both get updated then it's possible that the optimizer will pick the wrong one when using the statistic for cardinality.

I'd be more interested to know how often you find that - with auto-create stats turned on - SQL Server hasn't created a statistic that would yield better performance or estimates, and then suddenly the plans improve because of the statistic you decided to create (that SQL Server didn't bother). My guess is that would be pretty rare (unless you haven't really enabled auto-create stats for those customers, or maybe these are filtered statistics, though that isn't possible for your 2005 folks).

As for stats going stale in general, you may be able to implement Trace Flag 2371 - which improves the thresholds for engaging auto-stats - for some of your customers.

Am I correct in saying that statistics are only used when creating the execution plan for a stored procedure, and they are not used in the actual execution context?

No, what happens is that the execution plan for a stored procedure is cached. Assuming there is enough available memory to continue holding the plan, it won't change unless one of the following happens (from Execution Plan Caching and Reuse in the SQL Server documentation, emphasis added):

• Changes made to a table or view referenced by the query (ALTER TABLE and ALTER VIEW).
• Changes made to a single procedure, which would drop all plans for that procedure from the cache (ALTER PROCEDURE).
• Changes to any indexes used by the execution plan.
• Updates on statistics used by the execution plan, generated either explicitly from a statement, such as UPDATE STATISTICS, or generated automatically.
• Dropping an index used by the execution plan.
• An explicit call to sp_recompile.
• Large numbers of changes to keys (generated by INSERT or DELETE statements from other users that modify a table referenced by the query).
• For tables with triggers, if the number of rows in the inserted or deleted tables grows significantly.
• Executing a stored procedure using the WITH RECOMPILE option.

So if the statistics are updated, the cached plan will automatically take the new stats into account and be recompiled.

How do you prevent execution plans from growing stale when you have a hundred thousand rows being added a day?

One way is if there are a lot of updates to the table, as mentioned above. A few hundred thousand changed rows may satisfy this condition. But if you want to be sure or have more granular control: by updating your stats. You can allow SQL Server to auto create and manage statistics, or manually do it yourself. You can find more info on either method at SQL Server Auto Update and Auto Create Statistics Options. When/if you do a weekly rebuild of indexes, this will also trigger the plans to be updated as well. Do some testing to see what is most beneficial to you, as updating statistics too often may not yield any real performance results.

If we're updating statistics frequently to combat this issue, would it make sense to use the OPTION (RECOMPILE) hint on this stored procedure's query?

You don't need to use RECOMPILE, since based off the excerpt above you can see that the execution plan gets updated appropriately whenever new stats are available. You may be fine with an end of day statistics update (if you're truly concerned) but I don't think it's explicitly a need based on what you've said so far. Again, though, I'd test it to see what impact this may have on your stored procedure performance and plan accordingly.