Which is faster, EXISTS before or after the INSERT?

Question

I have an SP in SQL Server which runs hundreds of times a minute, and needs to check incoming traffic against a database. At the moment it does the following

INSERT INTO table
SELECT @value1,@value2 WHERE NOT EXISTS 
(SELECT * FROM table WHERE value1 = @value1 AND value2 = @value2);

However, I could also go with

IF NOT EXISTS(SELECT * FROM table WHERE value1 = @value1 AND value2 = @value2)    
   INSERT INTO table (value1,value2) VALUES (@value1,@value2);

Which would be faster? I get the feeling there's not much difference between them but I'm historically not very good at TSQL... =/

UPDATE: Whoops... meant to state that the EXISTS uses more than 1 value to find if a record exists, so a unique constraint won't work. Edited the sample to reflect that...

Answer 1

Both variants are incorrect. You will insert pairs of duplicate @value1, @value2, guaranteed.

The correct way to handle this is to enforce a unique constraint on two columns and to always INSERT and handle the constraint violation:

ALTER TABLE Table ADD CONSTRAINT uniqueValue1Value UNIQUE (value1, values2);

and to insert:

BEGIN TRY
   INSERT INTO Table (value1, value2) VALUES (@value1, @value2);
END TRY
BEGIN CATCH
   DECLARE @error_number int, @error_message NVARCHAR(4000), @xact_state INT;
   SET @error_number = ERROR_NUMBER();
   SET @error_message = ERROR_MESSAGE();
   SET @xact_state = XACT_STATE();
   IF (@xact_state = -1)
   BEGIN
     ROLLBACK TRANSACTION;
   END
   IF (@error_number != 2627) /* 2627 is ' Cannot insert duplicate key in object ...' */
   BEGIN
      RAISERROR(N'Error inserting into Table: %i %s', 16,1, @errror_number, @error_message);
   END
ENd CATCH

While these may seem complicated, one has to factor in a little detail named correctness. This is by far simpler when compared with a lock hints based solution. This is also the most performant solution: does only one seek. All other solutions need at least two seeks (one to validate that it can be inserted, one to insert).