Only inserting a row if it's not already there

Question

I had always used something similar to the following to achieve it:

INSERT INTO TheTable SELECT     @primaryKey,     @value1,     @value2 WHERE     NOT EXISTS     (SELECT         NULL     FROM         TheTable     WHERE         PrimaryKey = @primaryKey) 

...but once under load, a primary key violation occurred. This is the only statement which inserts into this table at all. So does this mean that the above statement is not atomic?

The problem is that this is almost impossible to recreate at will.

Perhaps I could change it to the something like the following:

INSERT INTO TheTable WITH     (HOLDLOCK,     UPDLOCK,     ROWLOCK) SELECT     @primaryKey,     @value1,     @value2 WHERE     NOT EXISTS     (SELECT         NULL     FROM         TheTable     WITH         (HOLDLOCK,         UPDLOCK,         ROWLOCK)     WHERE         PrimaryKey = @primaryKey) 

Although, maybe I'm using the wrong locks or using too much locking or something.

I have seen other questions on stackoverflow.com where answers are suggesting a "IF (SELECT COUNT(*) ... INSERT" etc., but I was always under the (perhaps incorrect) assumption that a single SQL statement would be atomic.

Does anyone have any ideas?

Answer 1

What about the "JFDI" pattern?

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

Seriously, this is quickest and the most concurrent without locks, especially at high volumes. What if the UPDLOCK is escalated and the whole table is locked?

Read lesson 4:

Lesson 4: When developing the upsert proc prior to tuning the indexes, I first trusted that the If Exists(Select…) line would fire for any item and would prohibit duplicates. Nada. In a short time there were thousands of duplicates because the same item would hit the upsert at the same millisecond and both transactions would see a not exists and perform the insert. After much testing the solution was to use the unique index, catch the error, and retry allowing the transaction to see the row and perform an update instead an insert.

Answer 2

I added HOLDLOCK which wasn't present originally. Please disregard the version without this hint.

As far as I'm concerned, this should be enough:

INSERT INTO TheTable  SELECT      @primaryKey,      @value1,      @value2  WHERE      NOT EXISTS      (SELECT 0      FROM TheTable WITH (UPDLOCK, HOLDLOCK)      WHERE PrimaryKey = @primaryKey)  

Also, if you actually want to update a row if it exists and insert if it doesn't, you might find this question useful.