Month: March 2016

Can I rely on SQL Server Identity values being in order?

It is best to not expect the identities to be consecutive because there are many scenarios that can leave gaps. It is better to consider the identity like an abstract number and to not attach any business meaning to it.

Basically, gaps can happen if you roll back INSERT operations (or explicitly delete rows), and duplicates can occur if you set the table property IDENTITY_INSERT to ON.

Gaps can occur when:
1. Records are deleted.
2. An error has occurred when attempting to insert a new record (rolled back)
3. An update/insert with explicit value (identity_insert option).
4. Incremental value is more than 1.

The identity property on a column has never guaranteed:
• Uniqueness
• Consecutive values within a transaction. If values must be consecutive then the transaction should use an exclusive lock on the table or use the SERIALIZABLE* isolation level.
• Consecutive values after server restart.
• Reuse of values.

If you cannot use identity values because of this, create a separate table holding a current value and manage access to the table and number assignment with your application. This does have the potential of impacting performance.

https://msdn.microsoft.com/en-us/library/ms186775(v=sql.105).aspx

https://msdn.microsoft.com/en-us/library/ms186775(v=sql.110).aspx

*A word of caution, using SERIALIZABLE will severely limit concurrency.  I would definitely test thoroughly first before implementing in production.

SERIALIZABLE

Specifies the following:

  •  Statements cannot read data that has been modified but not yet committed by other transactions.
  •  No other transactions can modify data that has been read by the current transaction until the current transaction completes.
  •  Other transactions cannot insert new rows with key values that would fall in the range of keys read by any statements in the current transaction until the current transaction completes.

Range locks are placed in the range of key values that match the search conditions of each statement executed in a transaction. This blocks other transactions from updating or inserting any rows that would qualify for any of the statements executed by the current transaction. This means that if any of the statements in a transaction are executed a second time, they will read the same set of rows. The range locks are held until the transaction completes. This is the most restrictive of the isolation levels because it locks entire ranges of keys and holds the locks until the transaction completes. Because concurrency is lower, use this option only when necessary. This option has the same effect as setting HOLDLOCK on all tables in all SELECT statements in a transaction.

Only one of the isolation level options can be set at a time, and it remains set for that connection until it is explicitly changed. All read operations performed within the transaction operate under the rules for the specified isolation level unless a table hint in the FROM clause of a statement specifies different locking or versioning behavior for a table.

If you issue SET TRANSACTION ISOLATION LEVEL in a stored procedure or trigger, when the object returns control the isolation level is reset to the level in effect when the object was invoked. For example, if you set REPEATABLE READ in a batch, and the batch then calls a stored procedure that sets the isolation level to SERIALIZABLE, the isolation level setting reverts to REPEATABLE READ when the stored procedure returns control to the batch.

The following example sets the TRANSACTION ISOLATION LEVEL for the session. For each Transact-SQL statement that follows, SQL Server holds all of the shared locks until the end of the transaction.

USE AdventureWorks2012;
GO
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
GO
BEGIN TRANSACTION;
GO
SELECT *     FROM HumanResources.EmployeePayHistory;
GO
SELECT *     FROM HumanResources.Department;
GO
COMMIT TRANSACTION;
GO

https://msdn.microsoft.com/en-us/library/ms173763.aspx

 

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