MySQL auto_increment列声明为float

Question

I need some help with a table InnoDB stored in a database MySQL version 5.5 on a Ubuntu linux server.

In this table there is a primat key column "id" that has the attribute auto increment and is declared as float.

Currently, the auto increment value stored in this table has reached high values (approximately, over 2 million).

Now, when I dump (or select) a range of records from this table I cannot see the correct value reached from the increment but a rounded value (for example: 2233937 becames 2233940 for the next records and so on with other records). This cause a malfunction when I import a MySQL dump file because the auto increment value is rounded and many records have the same incremental id.

A side note. In this database I have other tables with the same situation (colums primary key declared as float with the auto increment attribute ) but with a minor incremental values stored in its. And in these tables the stored auto increment value has the correct number.

Then, I have these question:

1) Why with this table I have this behavior? The column has reached the maximum size?

2) How I can resolve this issue with this table?

Thanks a lot for any response.

Lorenzo

UPDATE

Currently, I have adopt this method to solve the issue.

I have execute the command ALTER TABLE to modify the type of the column from float to integer and this seems to work.

Please, can you confirm to me that this is the correct way (or perhaps only one of many ways) to solve this problem?

Answer 1

As is well mentioned in the documentation:

B.5.4.8 Problems with Floating-Point Values

Floating-point numbers sometimes cause confusion because they are approximate and not stored as exact values.

A local test presents a couple of alternatives, but without guarantees (especially the second alternative):

Alternative 1:

mysql> DROP TABLE IF EXISTS `tbl_test`;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE IF NOT EXISTS `tbl_test` (
    ->   `id` FLOAT UNSIGNED AUTO_INCREMENT PRIMARY KEY
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW CREATE TABLE `tbl_test`\G
*************************** 1. row ***************************
       Table: tbl_test
Create Table: CREATE TABLE `tbl_test` (
  `id` float unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> INSERT INTO `tbl_test`
    ->   (`id`)
    -> VALUES
    ->   (2233937),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (16776999),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL);
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

mysql> SELECT `id`,
    ->        CONVERT(`id`, UNSIGNED)
    -> FROM `tbl_test`;
+----------+-------------------------+
| id       | CONVERT(`id`, UNSIGNED) |
+----------+-------------------------+
|  2233940 |                 2233937 |
|  2233940 |                 2233938 |
|  2233940 |                 2233939 |
|  2233940 |                 2233940 |
|  2233940 |                 2233941 |
|  2233940 |                 2233942 |
|  2233940 |                 2233943 |
|  2233940 |                 2233944 |
|  2233940 |                 2233945 |
|  2233950 |                 2233946 |
|  2233950 |                 2233947 |
|  2233950 |                 2233948 |
|  2233950 |                 2233949 |
|  2233950 |                 2233950 |
|  2233950 |                 2233951 |
|  2233950 |                 2233952 |
| 16777000 |                16776999 |
| 16777000 |                16777000 |
| 16777000 |                16777001 |
| 16777000 |                16777002 |
| 16777000 |                16777003 |
| 16777000 |                16777004 |
| 16777000 |                16777005 |
| 16777000 |                16777006 |
| 16777000 |                16777007 |
| 16777000 |                16777008 |
| 16777000 |                16777009 |
| 16777000 |                16777010 |
| 16777000 |                16777011 |
| 16777000 |                16777012 |
| 16777000 |                16777013 |
| 16777000 |                16777014 |
+----------+-------------------------+
32 rows in set (0.00 sec)

Example db-fiddle .

Alternative 2: (be careful)

mysql> DROP TABLE IF EXISTS `tbl_test`;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE IF NOT EXISTS `tbl_test` (
    ->   `id` FLOAT UNSIGNED AUTO_INCREMENT PRIMARY KEY
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW CREATE TABLE `tbl_test`\G
*************************** 1. row ***************************
       Table: tbl_test
Create Table: CREATE TABLE `tbl_test` (
  `id` float unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> INSERT INTO `tbl_test`
    ->   (`id`)
    -> VALUES
    ->   (2233937),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (16776999),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL),
    ->   (NULL);
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE `tbl_test` CHANGE `id`
    ->   `id` FLOAT(8, 0) UNSIGNED AUTO_INCREMENT;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE `tbl_test`\G
*************************** 1. row ***************************
       Table: tbl_test
Create Table: CREATE TABLE `tbl_test` (
  `id` float(8,0) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=16777016 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> SELECT `id`
    -> FROM `tbl_test`;
+----------+
| id       |
+----------+
|  2233937 |
|  2233938 |
|  2233939 |
|  2233940 |
|  2233941 |
|  2233942 |
|  2233943 |
|  2233944 |
|  2233945 |
|  2233946 |
|  2233947 |
|  2233948 |
|  2233949 |
|  2233950 |
|  2233951 |
|  2233952 |
| 16776999 |
| 16777000 |
| 16777001 |
| 16777002 |
| 16777003 |
| 16777004 |
| 16777005 |
| 16777006 |
| 16777007 |
| 16777008 |
| 16777009 |
| 16777010 |
| 16777011 |
| 16777012 |
| 16777013 |
| 16777014 |
+----------+
32 rows in set (0.00 sec)

Example db-fiddle .

Other information of interest:

• 11.2.3 Floating-Point Types (Approximate Value) - FLOAT, DOUBLE .
• 11.7 Data Type Storage Requirements::Numeric Type Storage Requirements .