Question

In my MySQL table I’ve created an ID column which I’m hoping to auto-increment in order for it to be the primary key.

I’ve created my table:

CREATE TABLE `test` (
        `id` INT( 11 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
        `name` VARCHAR( 50 ) NOT NULL ,
        `date_modified` DATETIME NOT NULL ,
    UNIQUE (
        `name`
    )
) TYPE = INNODB;

then Inserted my records:

INSERT INTO `test` ( `id` , `name` , `date_modified` ) 
VALUES (
    NULL , 'TIM', '2011-11-16 12:36:30'
), (
    NULL , 'FRED', '2011-11-16 12:36:30'
);

I’m expecting that my ID’s for the above are 1 and 2 (respectively). And so far this is true.
However when I do something like this:

insert into test (name) values ('FRED') 
on duplicate key update date_modified=now();

then insert a new record, I’m expecting it to be 3, however now I’m shown an ID of 4; skipping the place spot for 3.

Normally this wouldn’t be an issue but I’m using millions of records which have thousands of updates every day.. and I don’t really want to even have to think about running out of ID’s simply because I’m skipping a ton of numbers..

Anyclue to why this is happening?

MySQL version: 5.1.44

Thank you

Answer 1

Is it possible to change your key to unsigned bigint – 18,446,744,073,709,551,615 is a lot of records – thus delaying the running out of ID’s

Found this in mysql manual http://dev.mysql.com/doc/refman/5.1/en/example-auto-increment.html

Use a large enough integer data type for the AUTO_INCREMENT column to hold the
maximum sequence value you will need. When the column reaches the upper limit of
the data type, the next attempt to generate a sequence number fails. For example,
if you use TINYINT, the maximum permissible sequence number is 127. 
For TINYINT UNSIGNED, the maximum is 255.

More reading here http://dev.mysql.com/doc/refman/5.6/en/information-functions.html#function_last-insert-id it could be inferred that the insert to a transactional table is a rollback so the manual says “LAST_INSERT_ID() is not restored to that before the transaction”

What about for a possible solution to use a table to generate the ID’s and then insert into your main table as the PK using LAST_INSERT_ID();

From the manual:

Create a table to hold the sequence counter and initialize it:

mysql> CREATE TABLE sequence (id INT NOT NULL);
mysql> INSERT INTO sequence VALUES (0);

Use the table to generate sequence numbers like this:

mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1);
mysql> SELECT LAST_INSERT_ID();

The UPDATE statement increments the sequence counter and causes the next call to
LAST_INSERT_ID() to return the updated value. The SELECT statement retrieves that
value. The mysql_insert_id() C API function can also be used to get the value. 
See Section 20.9.3.37, “mysql_insert_id()”.