In tsql is an Insert with a Select statement safe in terms of concurrency?

Question

In my answer to this SO question I suggest using a single insert statement, with a select that increments a value, as shown below.

Insert Into VersionTable 
(Id, VersionNumber, Title, Description, ...) 
Select @ObjectId, max(VersionNumber) + 1, @Title, @Description 
From VersionTable 
Where Id = @ObjectId 

I suggested this because I believe that this statement is safe in terms of concurrency, in that if another insert for the same object id is run at the same time, there is no change of having duplicate version numbers.

Am I correct?

Answer 1

I think you're assumption is incorrect. When you query the VersionNumber table, you are only putting a read lock on the row. This does not prevent other users from reading the same row from the same table. Therefore, it is possible for two processes to read the same row in the VersionNumber table at the same time and generate the same VersionNumber value.

Answer 2

The default isolation of read commited makes this unsafe, if two of these run in perfect paralel you will get a duplicate since there is no read lock applied.

You need REPEATABLE READ or SERIALIZABLE isolation levels to make it safe.

Answer 3

• You need a unique constraint on (Id, VersionNumber) to enforce it

• I'd use ROWLOCK, XLOCK hints to block other folk reading the locked row where you calculate

• or wrap the INSERT in a TRY/CATCH. If I get a duplicate, try again...

Answer 4

As Paul writes: No, it's not safe, for which I would like to add empirical evidence: Create a table Table_1 with one field ID and one record with value 0. Then execute the following code simultaneously in two Management Studio query windows:

declare @counter int
set @counter = 0
while @counter < 1000
begin
  set @counter = @counter + 1

  INSERT INTO Table_1
    SELECT MAX(ID) + 1 FROM Table_1 

end

Then execute

SELECT ID, COUNT(*) FROM Table_1 GROUP BY ID HAVING COUNT(*) > 1

On my SQL Server 2008, one ID (662) was created twice. Thus, the default isolation level applied to single statements is not sufficient.

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EDIT: Clearly, wrapping the INSERT with BEGIN TRANSACTION and COMMIT won't fix it, since the default isolation level for transactions is still READ COMMITTED, which is not sufficient. Note that setting the transaction isolation level to REPEATABLE READ is also not sufficient. The only way to make the above code safe is to add

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

at the top. This, however, caused deadlocks every now and then in my tests.

EDIT: The only solution I found which is safe and does not produce deadlocks (at least in my tests) is to explicitly lock the table exclusively (default transaction isolation level is sufficient here). Beware though; this solution might kill performance:

...loop stuff...
    BEGIN TRANSACTION

    SELECT * FROM Table_1 WITH (TABLOCKX, HOLDLOCK) WHERE 1=0

    INSERT INTO Table_1
      SELECT MAX(ID) + 1 FROM Table_1 

    COMMIT
...loop end...