About lock behavior when update row in sql server - sql

Question

Now, I'm trying to increment number sequential in SQL Server with the number provided from users.
I have a problem when multiple user insert a row same time with same number.
I try to update the number that user provided to a temporary table, and I expect when I update the same table with same condition, SQL Server will lock any modified to this row until the current update finished, but it not.
Here is the update statement I used:
UPDATE GlobalParam
SET ValueString = (CAST(ValueString as bigint) + 1)
WHERE Id = 'xxxx'
Could you tell me any way to force the other update command wait until the current command finished ?
This is entire my command :
DECLARE #Result bigint;
UPDATE GlobalParam SET ValueString = (SELECT MAX(Code) FROM Item)
DECLARE #SelectTopStm nvarchar(MAX);
DECLARE #ExistRow int
SET #SelectTopStm = 'SELECT #ExistRow = 1 FROM (SELECT TOP 1 Code FROM Item WHERE Code = '999') temp'
EXEC sp_executesql #SelectTopStm, N'#ExistRow int output', #ExistRow output
IF (#ExistRow is not null)
BEGIN
DECLARE #MaxValue bigint
DECLARE #ReturnUpdateTbl table (ValueString nvarchar(max));
UPDATE GlobalParam SET ValueString = (CAST(ValueString as bigint) + 1)
OUTPUT inserted.ValueString INTO #ReturnUpdateTbl
WHERE [Id] = '333A8E1F-16DD-E411-8280-D4BED9D726B3'
SELECT TOP 1 #MaxValue = CAST(ValueString as bigint) FROM #ReturnUpdateTbl
SET #Result = #MaxValue
END
ELSE
BEGIN
SET #Output = 999
END
END
I write the codes above as a stored procedure.
Here is the real code when I insert one Item:
DECLARE #IncrementResult BIGINT
EXEC IncrementNumberUnique
, (some parameters)..
,#Result = #IncrementResult OUTPUT
INSERT INTO ITEM (Id, Code) VALUES ('xxxx', #IncrementResult)
I create 3 threads and make it run at the same time.
The return result :
Id Code
1 999
2 1000
3 1000
Thanks

Answer 1

If I understood your requirements, try ROWLOCK hint to tell the optimizer to start with locking the rows one by one as the update needs them.
UPDATE GlobalParam WITH(ROWLOCK)
SET ValueString = (CAST(ValueString as bigint) + 1)
WHERE Id = 'xxxx'

Answer 2

By default SQL Server does READ Committed locking which releases READ locks once the read operation is committed. Once the Update statement below is complete, all read locks are released from the table Item.
UPDATE GlobalParam SET ValueString = (SELECT MAX(Code) FROM Item)
Since your INSERT into Item is outside the scope of your procedure. you can run the thread in SERIALIZABLE isolation level. Something like this.
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
DECLARE #IncrementResult BIGINT
EXEC IncrementNumberUnique
, (some parameters)..
,#Result = #IncrementResult OUTPUT
INSERT INTO ITEM (Id, Code) VALUES ('xxxx', #IncrementResult)
Changing the isolation level to SERIALIZABLE will increase blocking and contention of resources on item table.
To know more about isolation level, refer this
you should look into identity columns and remove such manual computation of incremental columns if possible.