I have a table with an identity column and I want to reserve a block of ids which I can use for bulk inserting, whilst allowing inserts to still happen into that table.

Note this is part of a bulk insert of several tables, where those other tables relate to these ids via an FK. Therefore I need to block them out so I can prepare the relationships beforehand.

I've found a solution which works by taking a lock on the table in a transaction and then does the reseeding (which is pretty fast). But it looks a bit hacky to me - is there a generally accepted pattern for doing this?

create table dbo.test
    id bigint not null primary key identity(1,1),
    SomeColumn nvarchar(100) not null

Here's the code to block out (make room for) some ids:

declare @numRowsToMakeRoomFor int = 100


        SELECT  MAX(Id) FROM dbo.test WITH (  XLOCK, TABLOCK ) -- will exclusively lock the table whilst this tran is in progress, 
        --another instance of this query will not be able to pass this line until this instance commits

        --get the next id in the block to reserve
        DECLARE @firstId BIGINT = (SELECT IDENT_CURRENT( 'dbo.test' )  +1);

        --calculate the block range
        DECLARE @lastId BIGINT = @firstId + (@numRowsToMakeRoomFor -1);

        --reseed the table
        DBCC CHECKIDENT ('dbo.test',RESEED, @lastId);


select @firstId;

My code is batch processing blocks of data in chunks of about 1000. I have about a billion rows to insert in total. Everything is working fine - the database isn't the bottle neck, the batch processing itself is computationally expensive and requires me to add a couple of servers to run in parallel, so I need to accommodate more than one process "batch inserting" at the same time.

  • 2
    Why not use INSERT .. SELECT .. with OUTPUT? No locking, no reseeding. Just INSERT and get the IDs from OUTPUT so you can use them in the other tables. – ypercubeᵀᴹ Sep 25 '19 at 20:51
  • @ypercubeᵀᴹ sorry, I see I didn't know you can Bulk insert with output. I'm doing the bulk insert from code (c#) which connects remotely - I guess I could write out files and then use bulk insert from a file. This might be a bit tricky. – Daniel James Bryars Sep 25 '19 at 21:01
  • 3
    What is the expensive, producing/calculating the rows? Or do you already have the data in files and just need to insert them? Also: how many tables and what are the relationships involved? Just 2 tables with an FK, many tables with a star schema, many tables with complex schema (cycles, multiple paths, etc)? – ypercubeᵀᴹ Sep 25 '19 at 21:14

You can use procedure (introduced in SQL Server 2012):

To use it you need to create a SEQUENCE object and use it as a default value instead of IDENTITY column.

There is an example:


    AS int   
    START WITH 1  
    CACHE 10  

CREATE TABLE Test.ProcessEvents  
        DEFAULT (NEXT VALUE FOR Test.RangeSeq),  
    EventTime datetime NOT NULL DEFAULT (getdate()),  
    EventCode nvarchar(5) NOT NULL,  
    Description nvarchar(300) NULL  
) ;

   @range_first_value_output sql_variant ;  

EXEC sp_sequence_get_range  
@sequence_name = N'Test.RangeSeq'  
, @range_size = 4  
, @range_first_value = @range_first_value_output OUTPUT ;

SELECT @range_first_value_output; 

Documentation: sp_sequence_get_range

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