What is wrong with my Try Catch in T-SQL?

Question

I am using SQL Server 2008 and when I run this Statement in Management studio the Select statement in the Catch Block is executed as expected

BEGIN TRY
 INSE         


        

Answer 1

This behaviour happens if you previously had a table IDontExist and compiled a plan for it that is still in the cache then drop the table.

It also happens if you run the individual statement twice even without the table ever existing. The first run raises an error that is not caught. The second run (after the first plan is cached) succeeds.

/*Clear Cache*/
DBCC FREEPROCCACHE

GO

BEGIN TRY
 INSERT INTO IDontExist(ProductID)
 VALUES(1)
END TRY
BEGIN CATCH
SELECT 'There was an error! ' + ERROR_MESSAGE()
END CATCH

GO
/*Plan now Cached*/

SELECT query_plan
FROM   sys.dm_exec_cached_plans cp
       OUTER APPLY sys.dm_exec_sql_text(plan_handle) t
       OUTER APPLY sys.dm_exec_query_plan(plan_handle) qp
WHERE  t.text LIKE '%IDontExist%'
OPTION (RECOMPILE)

GO

BEGIN TRY
 INSERT INTO IDontExist(ProductID)
 VALUES(1)
END TRY
BEGIN CATCH
SELECT 'There was an error! ' + ERROR_MESSAGE()
END CATCH

GO

The INSERT statement gets auto parameterised.

If you change your Select * from IDontExist statement to Select * from IDontExist WHERE ProductID = 1 this also becomes auto parameterised and they behave the same.

I'm not absolutely certain why the auto parameterisation makes a difference here. I think that it is explained by the below extract from BOL however.

The following types of errors are not handled by a CATCH block when they occur at the same level of execution as the TRY…CATCH construct ... [those] that occur during statement-level recompilation ... If an error occurs during compilation or statement-level recompilation at a lower execution level (for example, when executing sp_executesql or a user-defined stored procedure) inside the TRY block, the error occurs at a lower level than the TRY…CATCH construct and will be handled by the associated CATCH block.

I presume the auto parametrization of that statement means that it gets recompiled at a lower execution level and is catchable.