Compressed columns with dictionaries

The per-column compression feature is a data type modifier, independent from user-level SQL and InnoDB data compression, that causes the data stored in the column to be compressed on writing to storage and decompressed on reading. For all other purposes, the data type is identical to the one without the modifier, i.e. no new data types are created. Compression is done by using the zlib library.

Additionally, it is possible to pre-define a set of strings for each compressed column to achieve a better compression ratio on relatively small individual data items.

This feature provides:

• a better compression ratio for text data which consists of a large number of predefined words (e.g. JSON or XML) using compression methods with static dictionaries

• a way to select columns in the table to compress (in contrast to the InnoDB row compression method). This feature is based on a patch provided by Weixiang Zhai.

Specifications

The feature is limited to InnoDB/XtraDB storage engine and to columns of the following data types:

• BLOB (including TINYBLOB, MEDIUMBLOB, LONGBLOG)

• TEXT (including TINYTEXT, MEDUUMTEXT, LONGTEXT)

• VARCHAR (including NATIONAL VARCHAR)

• VARBINARY

• JSON

A compressed column is declared by using the syntax that extends the existing COLUMN_FORMAT modifier: COLUMN_FORMAT COMPRESSED. If this modifier is applied to an unsupported column type or storage engine, an error is returned.

The compression can be specified:

• when creating a table: CREATE TABLE ... (..., foo BLOB COLUMN_FORMAT COMPRESSED, ...);

• when altering a table and modifying a column to the compressed format: ALTER TABLE ... MODIFY [COLUMN] ... COLUMN_FORMAT COMPRESSED, or ALTER TABLE ... CHANGE [COLUMN] ... COLUMN_FORMAT COMPRESSED.

Unlike Oracle MySQL, compression is applicable to generated stored columns. Use this syntax extension as follows:

mysql> CREATE TABLE t1(
       id INT,
       a BLOB,
       b JSON COLUMN_FORMAT COMPRESSED,
       g BLOB GENERATED ALWAYS AS (a) STORED COLUMN_FORMAT COMPRESSED WITH COMPRESSION_DICTIONARY numbers
     ) ENGINE=InnoDB;

To decompress a column, specify a value other than COMPRESSED to COLUMN_FORMAT: FIXED, DYNAMIC, or DEFAULT. If there is a column compression/decompression request in an ALTER TABLE, it is forced to the COPY algorithm.

Two new variables: innodb_compressed_columns_zip_level and innodb_compressed_columns_threshold have been implemented.

Compression dictionary support

To achieve a better compression ratio on relatively small individual data items, it is possible to predefine a compression dictionary, which is a set of strings for each compressed column.

Compression dictionaries can be represented as a list of words in the form of a string (a comma or any other character can be used as a delimiter although not required). In other words, a, bb, ccc, a bb ccc, and abbccc will have the same effect. However, the latter is more compact. The Quote symbol quoting is handled by regular SQL quoting. The maximum supported dictionary length is 32506 bytes (zlib limitation).

The compression dictionary is stored in a new system InnoDB table. As this table is of the data dictionary kind, concurrent reads are allowed, but writes are serialized, and reads are blocked by writes. Table read through old read views are not supported, similar to InnoDB internal DDL transactions.

Interaction with innodb_force_recovery variable

Compression dictionary operations are treated like DDL operations with the exception when innodb_force_value is set to 3: with values less than 3, compression dictionary operations are allowed, and with values >= 3, they are forbidden.

Example

In order to use the compression dictionary, you need to create it. This can be done by running:

mysql> SET @dictionary_data = 'one' 'two' 'three' 'four';

Expected output

Query OK, 0 rows affected (0.00 sec)

mysql> CREATE COMPRESSION_DICTIONARY numbers (@dictionary_data);

Expected output

Query OK, 0 rows affected (0.00 sec)

To create a table that has both compression and compressed dictionary support you should run:

mysql> CREATE TABLE t1(
        id INT,
        a BLOB COLUMN_FORMAT COMPRESSED,
        b BLOB COLUMN_FORMAT COMPRESSED WITH COMPRESSION_DICTIONARY numbers
      ) ENGINE=InnoDB;

The following example shows how to insert a sample of JSON data into the table:

SET @json_value =
'[\n'
' {\n'
' "one" = 0,\n'
' "two" = 0,\n'
' "three" = 0,\n'
' "four" = 0\n'
' },\n'
' {\n'
' "one" = 0,\n'
' "two" = 0,\n'
' "three" = 0,\n'
' "four" = 0\n'
' },\n'
' {\n'
' "one" = 0,\n'
' "two" = 0,\n'
' "three" = 0,\n'
' "four" = 0\n'
' },\n'
' {\n'
' "one" = 0,\n'
' "two" = 0,\n'
' "three" = 0,\n'
' "four" = 0\n'
' }\n'
']\n'
;

mysql> INSERT INTO t1 VALUES(0, @json_value, @json_value);
Query OK, 1 row affected (0.01 sec)

INFORMATION_SCHEMA Tables

This feature implements two new INFORMATION_SCHEMA tables.

INFORMATION_SCHEMA.COMPRESSION_DICTIONARY

Column Name	Description
‘BIGINT(21)_UNSIGNED dict_version’	‘dictionary version’
‘VARCHAR(64) dict_name’	‘dictionary name’
‘BLOB dict_data’	‘compression dictionary string’

This table provides a view of the internal compression dictionary. The SUPER privilege is required to query it.

INFORMATION_SCHEMA.COMPRESSION_DICTIONARY_TABLES

Column Name	Description
‘BIGINT(21)_UNSIGNED table_schema’	‘table schema’
‘BIGINT(21)_UNSIGNED table_name’	‘table ID from INFORMATION_SCHEMA.INNODB_SYS_TABLES’
‘BIGINT(21)_UNSIGNED column_name’	‘column position (starts from 0 as in INFORMATION_SCHEMA.INNODB_SYS_COLUMNS)’
‘BIGINT(21)_UNSIGNED dict_name’	‘dictionary ID’

This table provides a view over the internal table that stores the mapping between the compression dictionaries and the columns using them. The SUPER privilege is require to query it.

Limitations

Compressed columns cannot be used in indices (neither on their own nor as parts of composite keys).

Note

CREATE TABLE t2 AS SELECT \* FROM t1 will create a new table with a compressed column, whereas CREATE TABLE t2 AS SELECT CONCAT(a,'') AS a FROM t1 will not create compressed columns.

At the same time, after executing the CREATE TABLE t2 LIKE t1 statement, t2.a will have the COMPRESSED attribute.

ALTER TABLE ... DISCARD/IMPORT TABLESPACE is not supported for tables with compressed columns. To export and import tablespaces with compressed columns, you uncompress them first with: ALTER TABLE ... MODIFY ... COLUMN_FORMAT DEFAULT.

mysqldump command line parameters

By default, with no additional options, mysqldump will generate a MySQL compatible SQL output.

All /\*!50633 COLUMN_FORMAT COMPRESSED \*/ and /\*!50633 COLUMN_FORMAT COMPRESSED WITH COMPRESSION_DICTIONARY <dictionary> \*/ won’t be in the dump.

When a new option enable-compressed-columns is specified, all /\*!50633 COLUMN_FORMAT COMPRESSED \*/ will be left intact and all /\*!50633 COLUMN_FORMAT COMPRESSED WITH COMPRESSION_DICTIONARY <dictionary> \*/ will be transformed into /\*!50633 COLUMN_FORMAT COMPRESSED \*/. In this mode, the dump will contain the necessary SQL statements to create compressed columns, but without dictionaries.

When a new enable-compressed-columns-with-dictionaries option is specified, dump will contain all compressed column attributes and compression dictionary.

Moreover, the following dictionary creation fragments will be added before CREATE TABLE statements which are going to use these dictionaries for the first time.

/*!50633 DROP COMPRESSION_DICTIONARY IF EXISTS <dictionary>; */
/*!50633 CREATE COMPRESSION_DICTIONARY <dictionary>(...); */

Two new options add-drop-compression-dictionary and skip-add-drop-compression-dictionary will control if /\*!50633 DROP COMPRESSION_DICTIONARY IF EXISTS <dictionary> \*/ part from previous paragraph will be skipped or not. By default, add-drop-compression-dictionary the mode will be used.

When both enable-compressed-columns-with-dictionaries and --tab=<dir> (separate file for each table) options are specified, necessary compression dictionaries will be created in each output file using the following fragment (regardless of the values of add-drop-compression-dictionary and skip-add-drop-compression-dictionary options).

/*!50633 CREATE COMPRESSION_DICTIONARY IF NOT EXISTS <dictionary>(...); */

System variables

innodb_compressed_columns_zip_level

Option	Description
Command-line	Yes
Config file	Yes
Scope	Global
Dynamic	Yes
Data type	Numeric
Default	6
Range	0-9

This variable is used to specify the compression level used for compressed columns. Specifying 0 will use no compression, 1 the fastest, and 9 the best compression. The default value is 6.

innodb_compressed_columns_threshold

Option	Description
Command-line	Yes
Config file	Yes
Scope	Global
Dynamic	Yes
Data type	Numeric
Default	96
Range	1 - 2^64-1 (or 2^32-1 for 32-bit release)

By default, a value being inserted will be compressed if its length exceeds innodb_compressed_columns_threshold bytes. Otherwise, it will be stored in the raw (uncompressed) form.

Please also note that because of the nature of some data, the compressed representation can be longer than the original value. In this case, it does not make sense to store such values in compressed form as Percona Server for MySQL would have to waste both memory space and CPU resources for unnecessary decompression. Therefore, even if the length of such non-compressible values exceeds innodb_compressed_columns_threshold, they will be stored in an uncompressed form (however, an attempt to compress them will still be made).

This parameter can be tuned to skip unnecessary attempts of data compression for values that are known in advance by the user to have a bad compression ratio of their first N bytes.

Get expert help

If you need assistance, visit the community forum for comprehensive and free database knowledge, or contact our Percona Database Experts for professional support and services.

Community Forum Get a Percona Expert

---

Last update: 2024-10-30