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Info (Zip) - 4.0Mb Creating Compressed Tables

Compressed tables can be created in file-per-table tablespaces or in general tablespaces. Table compression is not available for the InnoDB system tablespace. The system tablespace (space 0, the .ibdata files) can contain user-created tables, but it also contains internal system data, which is never compressed. Thus, compression applies only to tables (and indexes) stored in file-per-table or general tablespaces.

Creating a Compressed Table in File-Per-Table Tablespace

To create a compressed table in a file-per-table tablespace, innodb_file_per_table must be enabled (the default). You can set this parameter in the MySQL configuration file (my.cnf or my.ini) or dynamically, using a SET statement.

After the innodb_file_per_table option is configured, specify the ROW_FORMAT=COMPRESSED clause or KEY_BLOCK_SIZE clause, or both, in a CREATE TABLE or ALTER TABLE statement to create a compressed table in a file-per-table tablespace.

For example, you might use the following statements:

SET GLOBAL innodb_file_per_table=1;
Creating a Compressed Table in a General Tablespace

To create a compressed table in a general tablespace, FILE_BLOCK_SIZE must be defined for the general tablespace, which is specified when the tablespace is created. The FILE_BLOCK_SIZE value must be a valid compressed page size in relation to the innodb_page_size value, and the page size of the compressed table, defined by the CREATE TABLE or ALTER TABLE KEY_BLOCK_SIZE clause, must be equal to FILE_BLOCK_SIZE/1024. For example, if innodb_page_size=16384 and FILE_BLOCK_SIZE=8192, the KEY_BLOCK_SIZE of the table must be 8. For more information, see Section, “General Tablespaces”.

The following example demonstrates creating a general tablespace and adding a compressed table. The example assumes a default innodb_page_size of 16K. The FILE_BLOCK_SIZE of 8192 requires that the compressed table have a KEY_BLOCK_SIZE of 8.

mysql> CREATE TABLESPACE `ts2` ADD DATAFILE 'ts2.ibd' FILE_BLOCK_SIZE = 8192 Engine=InnoDB;

  • As of MySQL 8.4, the tablespace file for a compressed table is created using the physical page size instead of the InnoDB page size, which makes the initial size of a tablespace file for an empty compressed table smaller than in previous MySQL releases.

  • If you specify ROW_FORMAT=COMPRESSED, you can omit KEY_BLOCK_SIZE; the KEY_BLOCK_SIZE setting defaults to half the innodb_page_size value.

  • If you specify a valid KEY_BLOCK_SIZE value, you can omit ROW_FORMAT=COMPRESSED; compression is enabled automatically.

  • To determine the best value for KEY_BLOCK_SIZE, typically you create several copies of the same table with different values for this clause, then measure the size of the resulting .ibd files and see how well each performs with a realistic workload. For general tablespaces, keep in mind that dropping a table does not reduce the size of the general tablespace .ibd file, nor does it return disk space to the operating system. For more information, see Section, “General Tablespaces”.

  • The KEY_BLOCK_SIZE value is treated as a hint; a different size could be used by InnoDB if necessary. For file-per-table tablespaces, the KEY_BLOCK_SIZE can only be less than or equal to the innodb_page_size value. If you specify a value greater than the innodb_page_size value, the specified value is ignored, a warning is issued, and KEY_BLOCK_SIZE is set to half of the innodb_page_size value. If innodb_strict_mode=ON, specifying an invalid KEY_BLOCK_SIZE value returns an error. For general tablespaces, valid KEY_BLOCK_SIZE values depend on the FILE_BLOCK_SIZE setting of the tablespace. For more information, see Section, “General Tablespaces”.

  • InnoDB supports 32KB and 64KB page sizes but these page sizes do not support compression. For more information, refer to the innodb_page_size documentation.

  • The default uncompressed size of InnoDB data pages is 16KB. Depending on the combination of option values, MySQL uses a page size of 1KB, 2KB, 4KB, 8KB, or 16KB for the tablespace data file (.ibd file). The actual compression algorithm is not affected by the KEY_BLOCK_SIZE value; the value determines how large each compressed chunk is, which in turn affects how many rows can be packed into each compressed page.

  • When creating a compressed table in a file-per-table tablespace, setting KEY_BLOCK_SIZE equal to the InnoDB page size does not typically result in much compression. For example, setting KEY_BLOCK_SIZE=16 typically would not result in much compression, since the normal InnoDB page size is 16KB. This setting may still be useful for tables with many long BLOB, VARCHAR or TEXT columns, because such values often do compress well, and might therefore require fewer overflow pages as described in Section, “How Compression Works for InnoDB Tables”. For general tablespaces, a KEY_BLOCK_SIZE value equal to the InnoDB page size is not permitted. For more information, see Section, “General Tablespaces”.

  • All indexes of a table (including the clustered index) are compressed using the same page size, as specified in the CREATE TABLE or ALTER TABLE statement. Table attributes such as ROW_FORMAT and KEY_BLOCK_SIZE are not part of the CREATE INDEX syntax for InnoDB tables, and are ignored if they are specified (although, if specified, they appear in the output of the SHOW CREATE TABLE statement).

  • For performance-related configuration options, see Section, “Tuning Compression for InnoDB Tables”.

Restrictions on Compressed Tables
  • Compressed tables cannot be stored in the InnoDB system tablespace.

  • General tablespaces can contain multiple tables, but compressed and uncompressed tables cannot coexist within the same general tablespace.

  • Compression applies to an entire table and all its associated indexes, not to individual rows, despite the clause name ROW_FORMAT.

  • InnoDB does not support compressed temporary tables. When innodb_strict_mode is enabled (the default), CREATE TEMPORARY TABLE returns errors if ROW_FORMAT=COMPRESSED or KEY_BLOCK_SIZE is specified. If innodb_strict_mode is disabled, warnings are issued and the temporary table is created using a non-compressed row format. The same restrictions apply to ALTER TABLE operations on temporary tables.