Sequence Numbers.; 10 minutes to read +1; In this article. APPLIES TO: SQL Server Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse A sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The code above creates a sequence object called seqperson, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access. To insert a new record into the 'Persons' table. SQL server can generate sequential GUIDs via a default invoking the newSequentialID function, so there no table of issued keys to maintain and no blocking bottleneck. This has given us unique IDs across the whole databases, across our entire enterprise actually as they are truly unique. APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse. You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.
Primary Key Generation Using Oracle's Sequence
Oracle provides the sequence utility to automatically generate unique primary keys. To use this utility to auto-generate primary keys for a CMP entity bean, you must create a sequence table and use the @AutomaticKeyGeneration annotation to point to this table.
In your Oracle database, you must create a sequence table that will create the primary keys, as shown in the following example:
This creates a sequences of primary key values, starting with 1, followed by 2, 3, and so forth. The sequence table in the example uses the default increment 1, but you can change this by specifying the increment keyword, such as increment by 3. When you do the latter, you must specify the exact same value in the cacheSize attribute of the @AutomaticKeyGeneration annotation:
If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @JarSettings Annotation. For more information on the definition of a CMP entity bean, see below.
Primary Key Generation Using SQL Server's IDENTITY
In SQL Server you can use the IDENTITY keyword to indicate that a primary-key needs to be auto-generated. The following example shows a common scenario where the first primary key value is 1, and the increment is 1:
In the CMP entity bean definition you need to specify SQLServer(2000) as the type of automatic key generator you are using. You can also provide a cache size:
If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @JarSettings Annotation. For more information on the definition of a CMP entity bean, see below.
Primary Key Generation Using a Named Sequence Table
A named sequence table is similar to the Oracle sequence functionality in that a dedicated table is used to generate primary keys. However, the named sequence table approach is vendor-neutral. To auto-generate primary keys this way, create a named sequence table using the two SQL statements shown in the example:
In the CMP entity bean definition you need to specify the named sequence table as the type of automatic key generator you are using. You can also provide a cache size:
If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @JarSettings Annotation. For more information on the definition of a CMP entity bean, see the next section.
Note. When you specify a cacheSize value for a named sequence table, a series of unique values are reserved for entity bean creation. When a new cache is necessary, a second series of unique values is reserved, under the assumption that the first series of unique values was entirely used. This guarantees that primary key values are always unique, although it leaves open the possibility that primary key values are not necessarily sequential. For instance, when the first series of values is 10..20, the second series of values is 21-30, even if not all values in the first series were actually used to create entity beans.
Defining the CMP Entity Bean
When defining a CMP entity bean that uses one of the primary key generators, you use the @AutomaticKeyGeneration annotation to point to the name of the primary key generator table to obtain primary keys. Also, you must define a primary key field of type Integer or Long to set and get the auto-generated primary key. However, the ejbCreate method does not take a primary key value as an argument. Instead the EJB container adds the correct primary key to the entity bean record.
The following example shows what the entity bean might look like. Notice that the bean uses the named sequence option described above, and that ejbCreate method does not take a primary key:Related Topics
-->Creates a sequence object and specifies its properties. A sequence is a user-defined schema bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The sequence of numeric values is generated in an ascending or descending order at a defined interval and can be configured to restart (cycle) when exhausted. Sequences, unlike identity columns, are not associated with specific tables. Applications refer to a sequence object to retrieve its next value. The relationship between sequences and tables is controlled by the application. User applications can reference a sequence object and coordinate the values across multiple rows and tables.
Unlike identity columns values that are generated when rows are inserted, an application can obtain the next sequence number without inserting the row by calling the NEXT VALUE FOR function. Use sp_sequence_get_range to get multiple sequence numbers at once.
For information and scenarios that use both CREATE SEQUENCE and the NEXT VALUE FOR function, see Sequence Numbers.
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Syntax
Arguments
sequence_name
Specifies the unique name by which the sequence is known in the database. Type is sysname.
[ built_in_integer_type user-defined_integer_type
A sequence can be defined as any integer type. The following types are allowed.
- tinyint - Range 0 to 255
- smallint - Range -32,768 to 32,767
- int - Range -2,147,483,648 to 2,147,483,647
- bigint - Range -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
- decimal and numeric with a scale of 0.
- Any user-defined data type (alias type) that is based on one of the allowed types.
If no data type is provided, the bigint data type is used as the default.
START WITH <constant>
The first value returned by the sequence object. The START value must be a value less than or equal to the maximum and greater than or equal to the minimum value of the sequence object. The default start value for a new sequence object is the minimum value for an ascending sequence object and the maximum value for a descending sequence object.
INCREMENT BY <constant>
Value used to increment (or decrement if negative) the value of the sequence object for each call to the NEXT VALUE FOR function. If the increment is a negative value, the sequence object is descending; otherwise, it is ascending. The increment cannot be 0. The default increment for a new sequence object is 1.
[ MINVALUE <constant> NO MINVALUE ]
Specifies the bounds for the sequence object. The default minimum value for a new sequence object is the minimum value of the data type of the sequence object. This is zero for the tinyint data type and a negative number for all other data types.
[ MAXVALUE <constant> NO MAXVALUE
Specifies the bounds for the sequence object. The default maximum value for a new sequence object is the maximum value of the data type of the sequence object.
[ CYCLE NO CYCLE ]
Property that specifies whether the sequence object should restart from the minimum value (or maximum for descending sequence objects) or throw an exception when its minimum or maximum value is exceeded. The default cycle option for new sequence objects is NO CYCLE.
Note
Cycling a SEQUENCE restarts from the minimum or maximum value, not from the start value.
[ CACHE [<constant> ] NO CACHE ]
Increases performance for applications that use sequence objects by minimizing the number of disk IOs that are required to generate sequence numbers. Defaults to CACHE.
For example, if a cache size of 50 is chosen, SQL Server does not keep 50 individual values cached. It only caches the current value and the number of values left in the cache. This means that the amount of memory required to store the cache is always two instances of the data type of the sequence object.
Note
If the cache option is enabled without specifying a cache size, the Database Engine will select a size. However, users should not rely upon the selection being consistent. Microsoft might change the method of calculating the cache size without notice.
When created with the CACHE option, an unexpected shutdown (such as a power failure) may result in the loss of sequence numbers remaining in the cache.
General Remarks
Sequence numbers are generated outside the scope of the current transaction. They are consumed whether the transaction using the sequence number is committed or rolled back. Duplicate validation only occurs once a record is fully populated. This can result in some cases where the same number is used for more than one record during creation, but then gets identified as a duplicate. If this occurs and other autonumber values have been applied to subsequent records, this can result in a gap between autonumber values and is expected behavior.
Cache management
To improve performance, SQL Server pre-allocates the number of sequence numbers specified by the CACHE argument.
For an example, a new sequence is created with a starting value of 1 and a cache size of 15. When the first value is needed, values 1 through 15 are made available from memory. The last cached value (15) is written to the system tables on the disk. When all 15 numbers are used, the next request (for number 16) will cause the cache to be allocated again. The new last cached value (30) will be written to the system tables.
If the Database Engine is stopped after you use 22 numbers, the next intended sequence number in memory (23) is written to the system tables, replacing the previously stored number.
After SQL Server restarts and a sequence number is needed, the starting number is read from the system tables (23). The cache amount of 15 numbers (23-38) is allocated to memory and the next non-cache number (39) is written to the system tables.
If the Database Engine stops abnormally for an event such as a power failure, the sequence restarts with the number read from system tables (39). Any sequence numbers allocated to memory (but never requested by a user or application) are lost. This functionality may leave gaps, but guarantees that the same value will never be issued two times for a single sequence object unless it is defined as CYCLE or is manually restarted.
The cache is maintained in memory by tracking the current value (the last value issued) and the number of values left in the cache. Therefore, the amount of memory used by the cache is always two instances of the data type of the sequence object.
Setting the cache argument to NO CACHE writes the current sequence value to the system tables every time that a sequence is used. This might slow performance by increasing disk access, but reduces the chance of unintended gaps. Gaps can still occur if numbers are requested using the NEXT VALUE FOR or sp_sequence_get_range functions, but then the numbers are either not used or are used in uncommitted transactions.
When a sequence object uses the CACHE option, if you restart the sequence object, or alter the INCREMENT, CYCLE, MINVALUE, MAXVALUE, or the cache size properties, it will cause the cache to be written to the system tables before the change occurs. Then the cache is reloaded starting with the current value (i.e. no numbers are skipped). Changing the cache size takes effect immediately.
CACHE option when cached values are available
The following process occurs every time that a sequence object is requested to generate the next value for the CACHE option if there are unused values available in the in-memory cache for the sequence object.
The next value for the sequence object is calculated.
The new current value for the sequence object is updated in memory.
The calculated value is returned to the calling statement.
CACHE option when the cache is exhausted
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The following process occurs every time a sequence object is requested to generate the next value for the CACHE option if the cache has been exhausted:
The next value for the sequence object is calculated.
The last value for the new cache is calculated.
The system table row for the sequence object is locked, and the value calculated in step 2 (the last value) is written to the system table. A cache-exhausted xevent is fired to notify the user of the new persisted value.
NO CACHE option
The following process occurs every time that a sequence object is requested to generate the next value for the NO CACHE option:
The next value for the sequence object is calculated.
The new current value for the sequence object is written to the system table.
The calculated value is returned to the calling statement.
Metadata
For information about sequences, query sys.sequences.
Security
Permissions
Requires CREATE SEQUENCE, ALTER, or CONTROL permission on the SCHEMA.
Members of the db_owner and db_ddladmin fixed database roles can create, alter, and drop sequence objects.
Members of the db_owner and db_datawriter fixed database roles can update sequence objects by causing them to generate numbers.
Sql Server Sequence Generated Primary Keyboard
The following example grants the user AdventureWorksLarry permission to create sequences in the Test schema.
Ownership of a sequence object can be transferred by using the ALTER AUTHORIZATION statement.
If a sequence uses a user-defined data type, the creator of the sequence must have REFERENCES permission on the type.
Audit
To audit CREATE SEQUENCE, monitor the SCHEMA_OBJECT_CHANGE_GROUP.
Examples
For examples of creating sequences and using the NEXT VALUE FOR function to generate sequence numbers, see Sequence Numbers.
Most of the following examples create sequence objects in a schema named Test.
To create the Test schema, execute the following statement.
A. Creating a sequence that increases by 1
In the following example, Thierry creates a sequence named CountBy1 that increases by one every time that it is used.
B. Creating a sequence that decreases by 1
The following example starts at 0 and counts into negative numbers by one every time it is used.
C. Creating a sequence that increases by 5
The following example creates a sequence that increases by 5 every time it is used.
D. Creating a sequence that starts with a designated number
After importing a table, Thierry notices that the highest ID number used is 24,328. Thierry needs a sequence that will generate numbers starting at 24,329. The following code creates a sequence that starts with 24,329 and increments by 1.
E. Creating a sequence using default values
The following example creates a sequence using the default values.
Sql Server Sequence Generated Primary Key Definition
Execute the following statement to view the properties of the sequence.
A partial list of the output demonstrates the default values.
start_value | -9223372036854775808 |
increment | 1 |
mimimum_value | -9223372036854775808 |
maximum_value | 9223372036854775807 |
is_cycling | 0 |
is_cached | 1 |
current_value | -9223372036854775808 |
F. Creating a sequence with a specific data type
The following example creates a sequence using the smallint data type, with a range from -32,768 to 32,767.
G. Creating a sequence using all arguments
The following example creates a sequence named DecSeq using the decimal data type, having a range from 0 to 255. The sequence starts with 125 and increments by 25 every time that a number is generated. Because the sequence is configured to cycle when the value exceeds the maximum value of 200, the sequence restarts at the minimum value of 100.
Execute the following statement to see the first value; the START WITH option of 125.
Execute the statement three more times to return 150, 175, and 200.
Execute the statement again to see how the start value cycles back to the MINVALUE option of 100.
Execute the following code to confirm the cache size and see the current value.
See Also
ALTER SEQUENCE (Transact-SQL)
DROP SEQUENCE (Transact-SQL)
NEXT VALUE FOR (Transact-SQL)
Sequence Numbers