Question

I have a question regarding locking in TSQL. Suppose I have a the following table:

A(int id, varchar name)

where id is the primary key, but is NOT an identity column.

I want to use the following pseudocode to insert a value into this table:

lock (A)
  uniqueID = GenerateUniqueID()  
  insert into A values (uniqueID, somename)
unlock(A)

How can this be accomplished in terms of T-SQL? The computation of the next id should be done with the table A locked in order to avoid other sessions to do the same operation at the same time and get the same id.

Answer 1

If you have custom logic that you want to apply in generating the ids, wrap it up into a user defined function, and then use the user defined function as the default for the column. This should reduce concurrency issue similarly to the provided id generators by deferring the generation to the point of insert and piggy backing on the insert locking behavior.

create table ids  (id int, somval varchar(20))
Go
Create function GenerateUniqueID()
returns int as 
Begin
    declare @ret int
    select @ret =  max(isnull(id,1)) * 2 from ids
    if @ret is null set @ret = 2  
    return @ret
End
go
alter table ids add Constraint DF_IDS Default(dbo.GenerateUniqueID())  for Id