June 16, 2010

The difference between estimated and actual plans

Optimization takes place before execution, so in one sense, any query plan is an estimated plan. But when you request an actual plan, SQL Server will actually execute each statement as it displays the plan, so the plan for subsequent statements can change after the earlier statements are executed. In addition, SQL Server adds additional information to the actual plan, after the execution takes place. As mentioned last time, the actual plan includes the number of processors used to execute the query. It will also include the number of rows returned from each step of the plan as it is executed, and the number of times each step is executed. If you are executing a plan that has already been cached and is now being reused, the actual plan will include both the parameter values the plan was originally compiled with and the parameter values used during execution.

i) You cannot use estimated plans for temp tables, you will get an error because the plan for the SELECT cannot be generated when the temp table does not exist

ii) One of the main reasons that the actual plan may different from the estimated plan is because of data changes to your data. In general (and assuming you have the option 'auto update statistics' enabled) if more than 20% of the data in a table changes, the optimizer will detect stale statistics and update them automatically. The updated statistics will then trigger a recompile.

iii) parameterised stored procedure - The first time you run the procedure, a estimated plan will be built based on the first parameter. The second time you run the procedure, the original plan will be used, and the statistics should show that SQL Server is performing more reads than there are pages in the table.
Seeing the different values used for compilation and execution can lead you to suspect an issue with parameter sniffing.

You can request that SQL Server display ESTIMATED plans without executing the query with any of the following options:

SET SHOWPLAN_TEXT ON

SET SHOWPLAN_ALL ON

SET SHOWPLAN_XML ON

For graphical estimated plans, you can use the Query | Display Estimated Execution Plan menu option. This can be invoked with a toolbar button, or with Cntl-L.

You can request that SQL Server display actual plans with any of the following options:

SET STATISTICS PROFILE ON

SET STATISTICS XML ON

1 comment:

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