Compressed columns with dictionaries

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 consist 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

The syntax to declare a compressed column is using an extension of an 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 pre-define 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 space-efficient. Quote symbol quoting is handled by regular SQL quoting. 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 unsupported, similarly to InnoDB internal DDL transactions.

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';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE COMPRESSION_DICTIONARY numbers (@dictionary_data);
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 implemented two new INFORMATION_SCHEMA tables.

table INFORMATION_SCHEMA.XTRADB_ZIP_DICT
Columns:
  • id (BIGINT(21)_UNSIGNED) – dictionary ID
  • name (VARCHAR(64)) – dictionary name
  • zip_dict (BLOB) – compression dictionary string

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

table INFORMATION_SCHEMA.XTRADB_ZIP_DICT_COLS
Columns:
  • table_id (BIGINT(21)_UNSIGNED) – table ID from INFORMATION_SCHEMA.INNODB_SYS_TABLES
  • column_pos (BIGINT(21)_UNSIGNED) – column position (starts from 0 as in INFORMATION_SCHEMA.INNODB_SYS_COLUMNS)
  • dict_id (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. 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 CREATE TABLE t2 LIKE t1 statement, t2.a will have COMPRESSED attribute.

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.

See also

How to find a good/optimal dictionary for zlib ‘setDictionary’ when processing a given set of data?
http://stackoverflow.com/questions/2011653/how-to-find-a-good-optimal-dictionary-for-zlib-setdictionary-when-processing-a
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