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Max + 1 duplicated !!!!

Author

Topic

mrsqlserver
Starting Member

6 Posts

Posted - 2013-04-12 : 15:06:00

Hi

I have this code in my SP, it was working fine till one day i had high traffic, now i get duplications on RecNo!!!
please i RecNo is not the Primary Key

Am i doing anything wrong here!
anyway i can fix this?

--------------------------------------------
DECLARE @MaxNo bigint
SELECT @MaxNo=Max(RecNo) + 1 FROM Records

INSERT INTO Records (RecNo, RecName, RecDate)
SELECT @MaxNo, @RecName, @RecDate
--------------------------------------------

russell
Pyro-ma-ni-yak

5072 Posts

Posted - 2013-04-12 : 15:47:30

[code]
BEGIN TRAN

DECLARE @MaxNo bigint
SELECT @MaxNo=Max(RecNo) + 1 FROM Records

INSERT INTO Records (RecNo, RecName, RecDate)
SELECT @MaxNo, @RecName, @RecDate

COMMIT[/code]
But a better way would be to make recNo an identity column and use scope_identity() to retrieve it

robvolk
Most Valuable Yak

15732 Posts

Posted - 2013-04-12 : 19:38:05

And regardless of which technique you use, without a unique or primary key constraint on RecNo column, you will still have duplicates.

Lamprey
Master Smack Fu Yak Hacker

4614 Posts

Posted - 2013-04-16 : 18:26:38

I agree with russell that identity would be better. But, if you are going implement hacks, besides a transaction, you might also want to a WITH (TABLOCKX) hint to your Recods table when you select the max.

LoztInSpace
Aged Yak Warrior

940 Posts

Posted - 2013-04-16 : 23:50:15

Pretty sure the transaction won't make any difference. You can still get the same answer in 2 sessions if you hit it at the right time.
Just use an identity and be done with it. The alternatives are messy, error prone and can drastically affect scalability and throughput because (assuming you get it right) you end up serialising all your transactions behind table locks.

russell
Pyro-ma-ni-yak

5072 Posts

Posted - 2013-04-17 : 01:20:32

quote:
Originally posted by LoztInSpace

Pretty sure the transaction won't make any difference.

100% sure that it will. Not maybe. It will.

SwePeso
Patron Saint of Lost Yaks

30421 Posts

Posted - 2013-04-17 : 01:35:44

quote:
Originally posted by mrsqlserver

--------------------------------------------
DECLARE @MaxNo bigint
SELECT @MaxNo=Max(RecNo) + 1 FROM Records

INSERT INTO Records (RecNo, RecName, RecDate)
SELECT @MaxNo, @RecName, @RecDate
--------------------------------------------

The first error is that the MAX part doesn't care about which RecName you are using. It just takes the MAX RecNo, regardless of which RecName is used. Is this by design or by error?
Should RecNo follow RecName?

N 56°04'39.26"
E 12°55'05.63"

LoztInSpace
Aged Yak Warrior

940 Posts

Posted - 2013-04-17 : 21:54:58

quote:
Originally posted by russell

quote:
Originally posted by LoztInSpace

Pretty sure the transaction won't make any difference.

100% sure that it will. Not maybe. It will.

I respectfully disagree. Now with added proof!
Do this:


create table records
(value int unique)
insert into records values(1)

Then run this code in 2 different sessions:


begin transaction

DECLARE @MaxNo bigint
SELECT @MaxNo=Max(value) + 1 FROM records
 select @MaxNo
WAITFOR DELAY '00:00:05'
INSERT INTO records (value)
SELECT @MaxNo

commit transaction

(1 row(s) affected)
Msg 2627, Level 14, State 1, Line 8
Violation of UNIQUE KEY constraint 'UQ__records__40BBEA3A36470DEF'. Cannot insert duplicate key in object 'records'. The duplicate key value is (2).
The statement has been terminated.

Because both sessions (correctly) read the same maximum value at the same time.

russell
Pyro-ma-ni-yak

5072 Posts

Posted - 2013-04-19 : 11:58:37

I think you're right. I stand corrected.

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