MySQL – Execution Plan Recompile

execution-planMySQL

I've added a new index to account for some performance issues however the queries that I would expect to be using the new index are still are not using it. In SQL Server you can flush an execution plan a couple of different ways to correct a poor performing query and I'm looking to understand if that is also possible in MySQL.

Can a specific execution plan be removed?
Can a all of the stored execution plans be removed?

Best Answer

Sadly, you cannot do that with MySQL.

Notwithstanding, there are two things try

1: Tweek the Query Optimizer

You can change the Query Optimizer behavior. Please see my posts about doing so, but always check the MySQL Documentation first and foremost.

2: Make MySQL use the index

Rather that reinvent the wheel, see my old post from Oct 28, 2011, Using FORCE INDEX

Related Solutions

How to Show Actual Query Costs in Execution Plan Pane in SSMS

I don't know of a way to do this in the plan from Management Studio, but this is one of the many things the free SentryOne Plan Explorer will do for you when you generate an actual plan from within the tool - it includes all the runtime metrics per statement.

SQL Server 2012 – Understanding Statistics, Execution Plans, and Ascending Key Problem

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.