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MySQL 5.5 Reference Manual  /  ...  /  Enabling Compression for a Table

14.12.2 Enabling Compression for a Table

Before creating a compressed table, make sure the innodb_file_per_table configuration option is enabled, and innodb_file_format is set to Barracuda. You can set these parameters in the MySQL configuration file my.cnf or my.ini, or with the SET statement without shutting down the MySQL server.

To enable compression for a table, you use the clauses ROW_FORMAT=COMPRESSED, KEY_BLOCK_SIZE, or both in a CREATE TABLE or ALTER TABLE statement.

To create a compressed table, you might use statements like these:

SET GLOBAL innodb_file_per_table=1;
SET GLOBAL innodb_file_format=Barracuda;
  • If you specify ROW_FORMAT=COMPRESSED, you can omit KEY_BLOCK_SIZE; the default compressed page size of 8KB is used.

  • If you specify KEY_BLOCK_SIZE, 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 additional performance-related configuration options, see Section 14.12.3, “Tuning Compression for InnoDB Tables”.

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 .ibd file of the table. 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.

Setting KEY_BLOCK_SIZE=16 typically does 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 14.12.5, “How Compression Works for InnoDB Tables”.

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, and are ignored if they are specified (although you see them in the output of the SHOW CREATE TABLE statement).

Restrictions on Compressed Tables

Because MySQL versions prior to 5.1 cannot process compressed tables, using compression requires specifying the configuration parameter innodb_file_format=Barracuda, to avoid accidentally introducing compatibility issues.

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

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

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