MySQL Cookbook

8.6 Changing a Table Type

8.6.1 Problem

A table has one type, and now you realize that another table type has properties that are more desirable for the way you use the table.

8.6.2 Solution

Use ALTER TABLE to change its type with a TYPE clause.

8.6.3 Discussion

MySQL supports several tables types, each of which have differing characteristics. Sometimes it's necessary or desirable to convert a table from one type to another. Some situations where a change of table type can be useful are as follows:

• Table conversions sometimes are done to gain access to features that are supported by one table type but not another. For example, ISAM tables do not allow NULL values in indexed columns. Also, AUTO_INCREMENT behavior in ISAM tables is such that sequence values may be non-monotonic under certain conditions. (See Chapter 11, for information about this.) You can convert an ISAM table to the MyISAM type, which does not suffer from these problems. Or you might find that you need to perform transactions on a table created using a type that doesn't provide transactional capabilities. To handle this problem, you can alter the table to a type such as InnoDB or BDB that does support transactions.

• The oldest table type supported by MySQL is ISAM, but ISAM tables are deprecated and at some point no longer will be supported. If you have ISAM tables, you should convert them at some point to another table type. Otherwise, after ISAM support is dropped, you'll be unable to upgrade to new releases of MySQL.

Changing a table type is easy; use ALTER TABLE with a TYPE specifier. For example, to convert a table to the MyISAM type, use this statement:

ALTER TABLE tbl_name TYPE = MYISAM;

To find out the current type of a table, use the SHOW TABLE STATUS statement (introduced in MySQL 3.23.0) or SHOW CREATE TABLE (introduced in MySQL 3.23.20):

mysql> SHOW TABLE STATUS LIKE 'mytbl'\G
*************************** 1. row ***************************
           Name: mytbl
           Type: MyISAM
     Row_format: Fixed
           Rows: 0
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 85899345919
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2002-07-15 21:28:34
    Update_time: 2002-07-15 21:28:34
     Check_time: NULL
 Create_options:
        Comment:
mysql> SHOW CREATE TABLE mytbl\G
*************************** 1. row ***************************
       Table: mytbl
Create Table: CREATE TABLE `mytbl` (
  `c` char(10) default NULL,
  `j` bigint(20) default NULL,
  `e2` enum('hardware','software','books','office supplies',
'telecommunications','furniture','utilities','shipping','tax') default NULL
) TYPE=MyISAM

Alternatively, use the mysqldump command-line utility:

% mysqldump --no-data cookbook mytbl
CREATE TABLE mytbl (
  c char(10) default NULL,
  j bigint(20) default NULL,
  e2 enum('hardware','software','books','office supplies',
'telecommunications','furniture','utilities','shipping','tax') default NULL
) TYPE=MyISAM;