concurrency - SQL Server Custom Identity Column -


i want generate custom identity column related type of product. can query guaranty order of identity , resolve concurrency. sample query:

 begin tran  insert tblkey values((select 'a-' + cast(max(cast(id int)) + 1 nvarchar) tblkey),'ehsan')  commit

try this:

begin tran  insert tblkey values((select max(id) + 1 nvarchar) tblkey (updlock)),'ehsan')  commit

when selecting max id acquire u lock on row. u lock incompatible u lock try acquire session same query running @ same time. 1 query executed @ given time. ids in order , continuous without gaps between them.

a better solution create table dedicated storing current or next id , use instead of maximum.

you can understand difference doing following:

prepare table

create table t(id int not null primary key clustered) insert t values(1)

and run following query in 2 different sessions 1 after less 10 seconds apart 

begin tran declare @idv int select @idv = max (id) t waitfor delay '0:0:10' insert t values(@idv+1) commit

wait while until both queries complete. observe 1 of them succeeded , other failed.

now same following query

begin tran declare @idv int select @idv = max (id) t (updlock) waitfor delay '0:0:5' insert t values(@idv+1) commit

view contents of t

cleanup t table drop table t


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