recompilation

How to avoid recompilation

- Don’t interleave DDL and DML statements.

- Avoid recompilation caused by statistics changes: use the KEEPFIXED PLAN option or disable the auto update statistics on the relevant table.

- Use table variables instead of temporary tables (as statistics are not created for table variables)

- Avoid changing SET options within the stored procedure.

- Use the OPTIMIZE FOR query hint that uses parameter values
supplied by you to compile the plan, regardless of the values of the parameter passed in by the calling application. It is recommended to use it in case you have a set of parameters that are "representative" from statistics point of view. This way you'll have a plan that works best most of the time.

CREATE PROCEDURE dbo.GetSales
@CustomerId INT
AS
SELECT ...
FROM
       dbo.Sales S
       INNER JOIN ...
WHERE S.CustomerId = @CustomerId
OPTION (OPTIMIZE FOR (@CustomerId = 1)) ;
GO

- Using plan guides. A plan guide allows you to use query hint or other optimization techniques without having to modify the query or procedure text. For example when the query is part of a third-party application and you are not able to modify it by including OPTION (OPTIMIZE FOR).
Example of creating a plan guide:

EXEC sp_create_plan_guide
       @name = N'MyGuide',
       @stmt = N'SELECT ...
                           FROM
                                 dbo.Sales S
                                 INNER JOIN ...
                                 WHERE S.CustomerId = @CustomerId'
,
       @type = N'OBJECT',
       @module_or_batch = N'dbo.CustomerList',
       @params = NULL,
       @hints = N'OPTION (OPTIMIZE FOR (@CustomerId = 1))' ;

Causes of recompilations

- The schema of regular tables, temporary tables, or views referred to in the stored procedure statement have changed. Schema changes include changes to the metadata of the table or the indexes on the table.

- Bindings (such as defaults) to the columns of regular or temporary tables have changed.

- Statistics on the table indexes or columns have changed past a certain threshold.

- An object did not exist when the stored procedure was compiled, but it was created during execution. This is called deferred object resolution.

- SET options have changed. (SET NOCOUNT doesn’t cause stored procedure recompilation)

- The execution plan was aged and deallocated.

- An explicit call of sp_recompile on a table, view, trigger or stored procedure

- There was an explicit use of the RECOMPILE hint:
CREATE PROCEDURE <spName> WITH RECOMPILE => prevents the caching of the stored procedure plan
EXEC <spname> WITH RECOMPILE; => a new plan is generated temporarily, without being cached of affecting any other existing cached plan
OPTION (RECOMPILE)</sql> => causes statement recompilation

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