Constraints check: TRY/CATCH vs Exists()

Question

I have a table with unique constraint on it:

create table dbo.MyTab
(
    MyTabID int primary key identity,
    SomeValue nvarchar(50)
);
Create Unique Index          


        

Answer 1

I've seen that article but note that for low failure rates I'd prefer the "JFDI" pattern. I've used this on high volume systems before (40k rows/second).

In Aaron's code, you can still get a duplicate when testing first under high load and lots of writes. (explained here on dba.se) This is important: your duplicates still happen, just less often. You still need exception handling and knowing when to ignore the duplicate error (2627)

Edit: explained succinctly by Remus in another answer

However, I would have a separate TRY/CATCH to test only for the duplicate error

BEGIN TRY

-- stuff

  BEGIN TRY
     INSERT etc
  END TRY
  BEGIN CATCH
      IF ERROR_NUMBER() <> 2627
        RAISERROR etc
  END CATCH

--more stuff

BEGIN CATCH
    RAISERROR etc
END CATCH