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From:Johnny Withers Date:March 9 2010 8:34pm
Subject:Re: UDF - Sequence Numbers
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After some tinkering around on one of my test database servers with this
sequence table design; I see what you mean.

Defining the table with store_id and seq_id columns allows me to UPDATE
table SET seq_id=LAST_INSERT_ID(seq_id+1) WHERE store_id=N and will block
any further updates on store_id=N to that table until the entire transaction
block is COMMIT'd. However, it does allow me to issue the same statement
with store_id=X and is not blocked.

I thought this design would allow for gaps in the sequence numbering, but it
works great.


On Mon, Mar 8, 2010 at 4:10 PM, Gavin Towey <gtowey@stripped> wrote:

> Others may correct me if I'm wrong, but if you want to maintain your own
> auto increment without duplicates like that, then you need to serialize
> inserts to your table.  Which means either doing an explicit table lock by
> the process that's inserting, or using innodb with transactions in the
> SERIALIZABLE tx isolation mode.
> If I were you, I would maintain your incrementing sequence in a separate
> table.
> Prep the table:
> CREATE TABLE store_seq ( store_id int unsigned not null, nextid int
> unsigned not null );
> insert into store_seq ( 1, 1 );
> Get next id:
> update store_seq set nextid=LAST_INSERT_ID(nextid+1) where store_id=1;
> select LAST_INSERT_ID();
> This will give you the next id in an atomic way, and avoid replication
> problems.  It means your insert will take two queries, but that's a small
> price to pay for correctness.
> Regards,
> Gavin Towey
> -----Original Message-----
> From: Johnny Withers [mailto:johnny@stripped]
> Sent: Monday, March 08, 2010 1:31 PM
> To: MySQL General List
> Subject: UDF - Sequence Numbers
> I have two servers, both running 5.0.77-log, one is setup as a master, the
> other as a replication slave.
> The database contains a table that holds records of loans for financial
> lending stores. This table has an ID column this is defined as auto
> increment. There is another column called "store_seq_num" that holds the
> sequence number for each loan done in each store. This column needs to work
> like the auto-increment field; however, it's value is dependent upon which
> store created the loan. Currently there is a UDF called fnNextStoreSeqNum
> that returns the next sequence number for the new loan for the given store.
> It does this by executing:
> SELECT MAX(store_seq_num)+1 AS NextId FROM trans_adv WHERE
> trans_adv.store_id=N;
> It uses the store_seq_num key and explain says "Select tables optimized
> away." in the extra column.
> The INSERT statement for this table looks something like this:
> INSERT INTO trans_adv(store_id,store_seq_num,...)
> VALUES(fnNextStoreSeqNum(10),10,....);
> The problem comes in on the replication server. Sometimes the sequence
> numbers do not match the master. The root cause of the problem seems to be
> when two loans are created in the same store at the same time (same second
> -- or even 2 seconds apart sometimes). The master duplicates the sequence
> number and the slave writes the correct sequence numbers. This seems to
> happen when the server is under heavy load (600+ queries per sec). I hvae a
> feeling it's due to the loan being created in a single transaction;
> therefore the sequence number for the "first" loan really didn't exist to
> any other connections until COMMIT was issued.
> Is there a better way to do these sequence numbers? Should the key be
> defined as UNIQUE? If it is defined as UNIQUE how can this key be added to
> the existing table that has duplicate sequence numbers?
> A partial create table statement is below for the trans_adv table.
> CREATE TABLE `trans_adv` (
>  `id` int(10) unsigned NOT NULL auto_increment,
>  `store_id` int(10) unsigned NOT NULL default '0',
>  `store_seq_num` int(10) unsigned NOT NULL default '0',
>  PRIMARY KEY  (`id`),
>  KEY `store_key` (`store_id`),
>  KEY `store_seq_num_key` (`company_id`,`store_id`,`store_seq_num`),
> ;
> --
> -----------------------------
> Johnny Withers
> 601.209.4985
> johnny@stripped
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Johnny Withers

UDF - Sequence NumbersJohnny Withers8 Mar
  • RE: UDF - Sequence NumbersGavin Towey8 Mar
    • Re: UDF - Sequence NumbersJohnny Withers9 Mar