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Advanced Stored Procedure Programming
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Identity Values and Triggers
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Unfortunately, the previous solution does not always work. SQL Server has a bug/feature that can change a value stored in the @@identity global variable. If the table in which the record was inserted (in this case, Inventory) has a trigger that inserts a record into some other table with an identity key, the value of that key will be recorded in @@identity. You can reproduce this behavior using the following script. It must be executed against the tempdb database.
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Create Table a (a_id int identity(1,1), a_desc varchar(20), b_desc varchar(20)) Go Create Table b (b_id int identity(1,1), b_desc varchar(20)) Go Create Trigger tr_a_I On dbo.a After Insert As If @@Rowcount = 0 Return Insert Into b (b_desc) Select b_desc from inserted Go -- For Insert
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Now execute this batch:
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Insert into b (b_desc) Values ('1') Insert into a (a_desc, b_desc)
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Values ('aaa', 'bbb') Select @@identity [IdentityValue]
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Query Analyzer returns the following result:
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(1 row(s) affected)
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(1 row(s) affected)
IdentityValue ---------------------------------------2 (1 row(s) affected)
The first Insert statement adds the first record to table b. The second Insert statement adds the first record in a table. Because there is a trigger on the table, another record (the second one) will be inserted into table b, and the value of @@identity will be set to 2. If there was no trigger, the Select statement would return a value of 1.
Sequence Number Table
Unfortunately, it is not easy to solve this problem. One solution is to create a table (for example, SequenceNumbers) that contains the highest sequence numbers for each table. So, each time that you want to insert a record into a table, you need to obtain a value from the sequence numbers table and increment that number by one. This value will then be used as a unique identifier (id) for the record that you want to insert.
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Advanced Stored Procedure Programming
This technique was a standard way to implement surrogate keys in earlier versions of SQL Server before identity values were introduced. Unfortunately, this technique is prone to concurrency contention problems, because there might be more processes competing to read, lock, and update a sequence key value. In earlier versions of SQL Server, it was not possible to lock a record, but only a page. A page could contain more than one record. Therefore, the process could lock a record even if the intent was to update some other record. This problem used to be solved by mechanically increasing the size of the record so that only one record could fit on a page. Dummy fields used to be added so that the size of the record became larger than half of the page (2K / 2 = 1K). This trick is called padding.
In SQL Server 2000 and SQL Server 7.0, there is no need for this because these versions automatically lock a record. However, processes can still compete to read, lock, and update a sequence key value in the same record. This can lead to a deadlock. The following stored procedure might be used to obtain an identifier from a table with sequence numbers:
Create Procedure prGetSequenceNumber -- return next Id for selected table -- and increment the value in SequenceNumbers table @chvTableName sysname, @intId int Output As -- read next Id Select @intId = SequenceNumber
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From SequenceNumbers Where Tablename = 'a table' -- increment SequenceNumber Update SequenceNumbers Set SequenceNumber = @intId + 1 Where Tablename = 'a table' Return
For example, it could happen that we have two processes on a server that need to insert a record into table a. One process might read a record from the SequenceNumbers table. Let s assume that the second process is just a little behind and that it manages to read a record before the first process can do anything else. Each of them places a shared lock on the record. Such a lock allows other processes to read the record but prevents them from updating it until the originating process finishes. Unfortunately, the first process cannot update this record any more because of the lock placed on it by the second process, and the second process cannot update the record because of the lock by the first process. Each will wait for the other process to give up. This situation is called a deadlock. SQL Server has a mechanism that will eventually kill one of the processes so that the other one can continue. The trouble is that the client application needs to execute everything again, and that valuable time has been lost. The standard way to avoid such deadlock is to place a hint in the From clause of the Select statement that will force SQL Server to put an update lock instead of a shared lock on a record. An update lock will prevent other processes from reading and putting locks on a record until the originating process is complete. Thus, the second process will wait until the first process is finished. Processes are thus serialized.
Create Procedure prGetSequenceNumber -- return next Id for selected table -- and increment the value in SequenceNumbers table @chvTableName sysname, @intId int OUTPUT
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