Understanding snapshot transactions

Snapshot transactions acquire write locks on updates, but read locks are never acquired for a transaction or statement that uses a snapshot. As a result, readers never block writers and writers never block readers, but writers can block writers if they attempt to update the same rows.

With snapshot isolation a transaction does not begin with a BEGIN TRANSACTION statement. Rather, it begins with the first read, insert, update, or delete within the transaction, depending on the snapshot isolation level being used for the transaction. The following example shows when a transaction begins for snapshot isolation:

SET OPTION PUBLIC.allow_snapshot_isolation = 'On';
         SET TEMPORARY OPTION isolation_level = 'snapshot';
   SELECT * FROM Products; --transaction begins and the statement only 
                           --sees changes that are already committed
   INSERT INTO Products 
         SELECT ID + 30, Name, Description,
         'Extra large', Color, 50, UnitPrice, NULL
         FROM Products
         WHERE Name = 'Tee Shirt';
COMMIT; --transaction ends