Question

I see an error message related to transaction isolation levels. There are two tables involved, first one is updated frequently with transaction isolation level set to SERIALIZABLE, the second one has a foreign key on first one.

Problem occurs when doing insert or update of the second table. Once in few hours I get the following error message:

Snapshot isolation transaction aborted due to update conflict. You cannot use snapshot isolation to access table ‘dbo.first’ directly or indirectly in database ‘DB’ to update, delete, or insert the row that has been modified or deleted by another transaction. Retry the transaction or change the isolation level for the update/delete statement.

I don’t set transaction isolation level when inserting or updating second table, also I ran command DBCC USEROPTIONS and it returns read_committed.

Answer 1

First:
It seems, you’re not using SERIALIZABLE, but snapshot isolation which was introduced with MSSQL 2005. Here is an article to understand the difference:
http://blogs.msdn.com/b/craigfr/archive/2007/05/16/serializable-vs-snapshot-isolation-level.aspx

=> This was based on the error, message, but as you have explained again in the comments the error comes when editing the second table.

Second:
For modifications MSSQL Server always tries to acquire locks, and since there are locks (by using a transaction) on the first table which escalate to locks on the second table because of the (foreign key) the operation fails. So every modification causes in fact a mini transaction.

The default transaction level on MSSQL is READ COMMITTED, but if you turn on the option READ_COMMITTED_SNAPSHOT it will convert READ COMMITTED to a SNAPSHOT like transaction every time you use READ COMMITTED. Which then leads to the error message you get.

To be precise as VladV pointed out, it’s not really using the SNAPSHOT isolation level, but READ COMMITTED with row versioning rather than locking, but only on a statement basis, where SNAPSHOT is using row versioning on a transaction basis.

To understand the difference check out this:
http://msdn.microsoft.com/en-us/library/ms345124(SQL.90).aspx

To find out more about the READ_COMMITTED_SNAPSHOT, its explained in detail here:
http://msdn.microsoft.com/en-us/library/tcbchxcb(VS.80).aspx
and here:
Default SQL Server IsolationLevel Changes

Another reason for you to see SNAPSHOT isolation if you have not specified it, is by using implicit transaction. After turing this option on and you don’t actually specify the isolation level on a modifying statement (which you don’t), MS SQL server will choose whatever he believes is the right isolation level. Here are the details:
http://msdn.microsoft.com/en-us/library/ms188317(SQL.90).aspx

For all theses scenarios the solution is the same though.

Solution:
You need to execute the operations in sequence, and you can do this by specifically using a transaction with SERIALIZABLE isolation level on both operations: when inserting/updating the first and when inserting/updating the second.
This way you block the respective other until it is completed.