SQL Server Sequence Number
Simulating Sequence Objects in SQL Server
Many applications need sequentially incremental number as unique/primary key of records. SQL Server 2005 today supports identity column as the primary mean to general sequence number, which generates the sequence number upon the execution of DML (insert) or bulk insert. The value of the insertion is known (when using insertion DML), post the execution of the DML. In some implementations, the sequence numbers (or next unique incremental number) need to be generated prior to the DML operation. For example, if sequence number can be retrieved prior to the DML operation, it can be easily implemented in situations where multiple DML (inserts, or even updates) using the same sequence number can be performed, and data can be insert into different tables, for example, parent-child, or based on the sequence value, data can be insert into different tables.
SQL Server 2005 Migration Assistant (SSMA) provides a different mechanism for sequence number generation. SSMA’s approach is to use a single table to hold all the sequence numbers. Each sequence number object is represented by a single row of record, which holds the property of the sequence, e.g. sequence name, current value, increments…etc. The generation of the next value is invoked by an extended stored procedure in order to minimize the impact on application concurrency – the execution of extended procedure is not within the transaction scope of the DML statement, and it holds very short duration locks on the row that represents the sequence number.
This BLOG describes two more methods of sequence number generation.
Option 1
This option is easy to use and maintain, but it is suitable only with lower volume invocations. It is also one of the widely used approaches, and often misused. This method uses a single table to hold all the sequences needed for the application, and each sequence is represented by one single record, thus serializes operation requesting next value of the same sequence number.
1. Create the following objects
Table AllSequences holds the information of all the sequences. As a simple example, the table and subsequent stored procedure is kept quite simple. Further enhancement is certainly possible based on the needs – for example, additional information of sequence such as maximum range, minimum range, loopback(yes/no) can be added.
· SeqName – name of the sequence
· Seed – seed value of the sequence
· Incr – incremental value of the sequence
· Currval – current value of the sequence
Create table AllSequences (
SeqName nvarchar(255) primary key, -- name of the sequence
Seed int not null default(1), -- seed value
Incr int not null default(1), -- incremental
Currval int
)
Go
The following stored procedure is used to create a sequence. The following input parameter is expected:
· SeqName – name of the sequence
· Seed – seed value of the sequence
· Incr – incremental value of the sequence
create procedure usp_CreateNewSeq
@SeqName nvarchar(255),
@seed int = 0,
@incr int = 1
as
begin
declare @currval int
if exists (
select 1 from AllSequences
where SeqName = @SeqName )
begin
print 'Sequence already exists.'
return 1
end
if @seed is null set @seed = 1
if @incr is null set @incr = 1
set @currval = @seed
insert into AllSequences (SeqName, Seed, Incr, CurrVal)
values (@SeqName, @Seed, @Incr, @CurrVal)
end
go
The following stored procedure is used to generate the next value of the specified sequence through input parameter SeqName.
create procedure usp_GetNewSeqVal
@SeqName nvarchar(255)
as
begin
declare @NewSeqVal int
set NOCOUNT ON
update AllSequences
set @NewSeqVal = CurrVal = CurrVal+Incr
where SeqName = @SeqName
if @@rowcount = 0 begin
print 'Sequence does not exist'
return
end
return @NewSeqVal
end
go
2. To create the new sequence, for example:
usp_CreateNewSeq N'TestSeq'
3. To retrieving the new sequence value, for example for sequence “TestSeq”
usp_GetNewSeqVal N'TestSeq'
One thing to be careful is to keep the execution of the stored procedure usp_CreateNewSeq outside the scope of subsequent transaction which uses the sequence number generated. This can minimize impact of blocking.
Typical usage
Declare @NewSeqVal int
Execute @NewSeqVal=usp_GetNewSeqVal @seqname=N'TestSeq'
print @NewSeqVal
Option 2
This method is intended for higher-throughput environments – such as thousands of transactions per second. A table with identity column is created for every sequence. Since the generation of identity value is lightweight, and not within the transaction scope of subsequent DML, this method can significantly improve the concurrency over the first method. In addition, it also consumes less resource than extended procedure.
1. Create one table with the following structure for each sequence, assuming seed 1, with increment of 1).
create table <tablename> (
SeqID int identity(1,1) primary key,
SeqVal varchar(1)
)
2. Create the following stored procedure. A delete operation is performed to remove the rows in the sequence table to reduce maintenance. This obviously adds some load to the transaction however it should be quite minimal. As an alternative, for higher throughput, the delete statement can be removed and a maintenance job can be scheduled during off-hour to purge the records in the table (truncate should not be used since it resets the identity value).
create procedure GetNewSeqVal_<tablename>
as
begin
declare @NewSeqValue int
set NOCOUNT ON
insert into <tablename> (SeqVal) values ('a')
set @NewSeqValue = scope_identity()
delete from <tablename> WITH (READPAST)
return @NewSeqValue
end
3. To retrieve the new value in a result set, simply execute the following:
GetNewSeqVal_<tablename>
Typical usage
Declare @NewSeqVal int
Exec @NewSeqVal = GetNewSeqVal_<tablename>
Among the two options described above, Option 1 is widely used in different variations but prone to blocking/deadlocking. Option 2 improves concurrency significantly with a simple approach.
Howard Yin
Comments
Anonymous
April 10, 2006
Very useful! But ... why can't SQL Server 2005 just implement sequences natively ? That would make life so much simpler, and avoid the need for workarounds such as this. Now if you could get it into 2005 SP1, that would be great :-).Anonymous
April 12, 2006
I'm a bit confused about Option 2. Why did you add the second column? Why not just have a single column and do:
INSERT <tablename>
DEFAULT VALUES
?
Does the additional column help with something in this scenario?Anonymous
April 13, 2006
The method you described certainly will work just fine. The key of this post is to perform the generation of sequence number prior to DML inserts through an identity column based dummy table to improve concurrency.
There can be many variation, and enhancements in different implementations. The sample code is just for illustrative purpose :)
One option can be using the extra field (of course a larger size) to hold additional information of the sequence, such as min/max, and loopback flag - through a dummy record. And the stored procedure "GetNewSeqVal..." can be enhanced to perform some checks.Anonymous
December 08, 2006
SQL Server 2005 includes a lot of changes to help make moving from Oracle to SQL Server easier, theseAnonymous
December 11, 2006
You can avoid accumulating data in the sequence table by putting the INSERT statement inside a nested transaction with a save point that you then rollback. begin tran save tran seq insert into <tablename> default values set @NewSeqValue = scope_identity() rollback tran seq commitAnonymous
July 06, 2010
The comment has been removedAnonymous
July 18, 2011
Now, It is in SQL Server Denali...:) Sequence ObjectAnonymous
August 24, 2011
very usefull, it will be better and simpler if sql server uses sequences types.Anonymous
December 10, 2011
how to implement, managing sequences and performance which is better identity or sequences please use the below link sqljunkieshare.com/.../sequences-in-sql-server-2012-implementingmanaging-performanceAnonymous
April 06, 2012
The comment has been removedAnonymous
October 03, 2012
Sequence feature which has been in Oracle for a number of years is now available in MS SQL 2012. A Sequence object is an object that provides functionality similar to Identity (Autonumber) column The sequence object can be used with more than one table which is not possible in identity object. This is useful when you have parent-child tables and you want to know the value before you insert records. Kash Mughal Senior DBA and Trainer www.sqlserver2012tutorial.comAnonymous
January 22, 2014
how would you cretae a function that returns sequential number?