Compressed tables are stored in a format that previous versions of InnoDB cannot process. To preserve downward compatibility of database files, compression can be specified only when the Barracuda data file format is enabled using the configuration parameter innodb_file_format.

Table compression is also not available for the InnoDB system tablespace. The system tablespace (space 0, the ibdata* files) may contain user data, but it also contains internal InnoDB system information, and therefore is never compressed. Thus, compression applies only to tables (and indexes) stored in their own tablespaces.

To use compression, enable the file-per-table mode using the configuration parameter innodb_file_per_table and enable the Barracuda disk file format using the parameter innodb_file_format. If necessary, you can set these parameters in the MySQL option file my.cnf or my.ini, or with the SET statement without shutting down the MySQL server.

Specifying ROW_FORMAT=COMPRESSED or KEY_BLOCK_SIZE in CREATE TABLE or ALTER TABLE statements produces these warnings if the Barracuda file format is not enabled. You can view them with the SHOW WARNINGS statement.

The “non-strict” behavior is intended to permit you to import a mysqldump file into a database that does not support compressed tables, even if the source database contained compressed tables. In that case, MySQL creates the table in ROW_FORMAT=COMPACT instead of preventing the operation.

When you import the dump file into a new database, if you want to have the tables re-created as they exist in the original database, ensure the server is running the InnoDB storage engine with the proper settings for the configuration parameters innodb_file_format and innodb_file_per_table,

The attribute KEY_BLOCK_SIZE is permitted only when ROW_FORMAT is specified as COMPRESSED or is omitted. Specifying a KEY_BLOCK_SIZE with any other ROW_FORMAT generates a warning that you can view with SHOW WARNINGS. However, the table is non-compressed; the specified KEY_BLOCK_SIZE is ignored).

If you are running in InnoDB strict mode, the combination of a KEY_BLOCK_SIZE with any ROW_FORMAT other than COMPRESSED generates an error, not a warning, and the table is not created.

Table 14.4, “Meaning of CREATE TABLE and ALTER TABLE options” summarizes how the various options on CREATE TABLE and ALTER TABLE are handled.

Table 14.5, “CREATE/ALTER TABLE Warnings and Errors when InnoDB Strict Mode is OFF” summarizes error conditions that occur with certain combinations of configuration parameters and options on the CREATE TABLE or ALTER TABLE statements, and how the options appear in the output of SHOW TABLE STATUS.

When InnoDB strict mode is OFF, InnoDB creates or alters the table, but may ignore certain settings, as shown below. You can see the warning messages in the MySQL error log. When InnoDB strict mode is ON, these specified combinations of options generate errors, and the table is not created or altered. You can see the full description of the error condition with SHOW ERRORS. For example:

mysql> CREATE TABLE x (id INT PRIMARY KEY, c INT)

-> ENGINE=INNODB KEY_BLOCK_SIZE=33333;

ERROR 1005 (HY000): Can't create table 'test.x' (errno: 1478)

mysql> SHOW ERRORS;
+-------+------+-------------------------------------------+ 
| Level | Code | Message                                   | 
+-------+------+-------------------------------------------+ 
| Error | 1478 | InnoDB: invalid KEY_BLOCK_SIZE=33333.     | 
| Error | 1005 | Can't create table 'test.x' (errno: 1478) | 
+-------+------+-------------------------------------------+ 
2 rows in set (0.00 sec)

When InnoDB strict mode is ON (innodb_strict_mode=1), the InnoDB storage engine rejects invalid ROW_FORMAT or KEY_BLOCK_SIZE parameters. For compatibility with earlier versions of InnoDB, strict mode is not enabled by default; instead, InnoDB issues warnings (not errors) for ignored invalid parameters.

Note that it is not possible to see the chosen KEY_BLOCK_SIZE using SHOW TABLE STATUS. The statement SHOW CREATE TABLE displays the KEY_BLOCK_SIZE (even if it was ignored by InnoDB). The real compressed page size inside InnoDB cannot be displayed by MySQL.