Compressed columns with dictionaries¶

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 (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. 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.

mysql> SET @dictionary_data = 'one' 'two' 'three' 'four';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE COMPRESSION_DICTIONARY numbers (@dictionary_data);
Query OK, 0 rows affected (0.00 sec)

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

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.

table INFORMATION_SCHEMA.COMPRESSION_DICTIONARY¶

Columns:	dict_version (BIGINT(21)_UNSIGNED) – dictionary version dict_name (VARCHAR(64)) – dictionary name dict_data (BLOB) – compression dictionary string

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

table INFORMATION_SCHEMA.COMPRESSION_DICTIONARY_TABLES¶

Columns:	table_schema (BIGINT(21)_UNSIGNED) – table schema table_name (BIGINT(21)_UNSIGNED) – table ID from INFORMATION_SCHEMA.INNODB_SYS_TABLES column_name (BIGINT(21)_UNSIGNED) – column position (starts from 0 as in INFORMATION_SCHEMA.INNODB_SYS_COLUMNS) dict_name (BIGINT(21)_UNSIGNED) – 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).

ALTER TABLE ... DISCARD/IMPORT TABLESPACE is not supported for tables with compressed columns. To export and import tablespaces with compressed columns, you need to 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 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>(...); */

Version Specific Information¶

• 8.0.13-3 Feature ported from Percona Server for MySQL 5.7.

System Variables¶

variable innodb_compressed_columns_zip_level¶

Command Line:	Yes
Config File:	Yes
Scope:	Global
Dynamic:	Yes
Variable Type:	Numeric
Default Value:	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. Default value is 6.

variable innodb_compressed_columns_threshold¶

Command Line:	Yes
Config File:	Yes
Scope:	Global
Dynamic:	Yes
Variable Type:	Numeric
Default Value:	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 raw (uncompressed) form.

Please also notice that because of the nature of some data, its 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 in order to skip unnecessary attempts of data compression for values that are known in advance by the user to have bad compression ratio of their first N bytes.