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Storage-Engine Independent Column Compression

Storage-engine independent column compression enables TINYBLOB, BLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT, VARCHAR and VARBINARY columns to be compressed.

This is performed by means of a new COMPRESSED column attribute:COMPRESSED[=<compression_method>]

Currently the only supported compression method is zlib.

Field Length Compatibility

When using the COMPRESSED attribute, note that FIELD LENGTH is reduced by 1; for example, a BLOB has a length of 65535, while BLOB COMPRESSED has 65535-1. See MDEV-15592.

New System Variables

column_compression_threshold

  • Description: Minimum column data length eligible for compression.

  • Commandline: --column-compression-threshold=#

  • Scope: Global, Session

  • Dynamic: Yes

  • Data Type: numeric

  • Default Value: 100

  • Range: 0 to 4294967295

column_compression_zlib_level

  • Description: zlib compression level (1 gives best speed, 9 gives best compression).

  • Commandline: --column-compression-zlib-level=#

  • Scope: Global, Session

  • Dynamic: Yes

  • Data Type: numeric

  • Default Value: 6

  • Range: 1 to 9

column_compression_zlib_strategy

  • Description: The strategy parameter is used to tune the compression algorithm. Use the value DEFAULT_STRATEGY for normal data, FILTERED for data produced by a filter (or predictor), HUFFMAN_ONLY to force Huffman encoding only (no string match), or RLE to limit match distances to one (run-length encoding). Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better. The effect of FILTERED is to force more Huffman coding and less string matching; it is somewhat intermediate between DEFAULT_STRATEGY and HUFFMAN_ONLY. RLE is designed to be almost as fast as HUFFMAN_ONLY, but give better compression for PNG image data. The strategy parameter only affects the compression ratio but not the correctness of the compressed output even if it is not set appropriately. FIXED prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.

  • Commandline: --column-compression-zlib-strategy=#

  • Scope: Global, Session

  • Dynamic: Yes

  • Data Type: enum

  • Default Value: DEFAULT_STRATEGY

  • Valid Values: DEFAULT_STRATEGY, FILTERED, HUFFMAN_ONLY, RLE, FIXED

column_compression_zlib_wrap

  • Description: If set to 1 (0 is default), generate zlib header and trailer and compute adler32 check value. It can be used with storage engines that don't provide data integrity verification to detect data corruption.

  • Commandline: --column-compression-zlib-wrap{=0|1}

  • Scope: Global, Session

  • Dynamic: Yes

  • Data Type: boolean

  • Default Value: OFF

New Status Variables

Column_compressions

  • Description: Incremented each time field data is compressed.

  • Scope: Global, Session

  • Data Type: numeric

Column_decompressions

  • Description: Incremented each time field data is decompressed.

  • Scope: Global, Session

  • Data Type: numeric

Limitations

  • The only supported method currently is zlib.

  • The CSV storage engine stores data uncompressed on-disk even if the COMPRESSED attribute is present.

  • It is not possible to create indexes over compressed columns.

Comparison with InnoDB Page Compression

Storage-independent column compression is different to InnoDB Page Compression in a number of ways.

  • It is storage engine independent, while InnoDB page compression applies to InnoDB only.

  • By being specific to a column, one can access non-compressed fields without the decompression overhead.

  • Only zlib is available, while InnoDB page compression can offer alternative compression algorithms.

  • It is not recommended to use multiple forms of compression over the same data.

  • It is intended for compressing large blobs, while InnoDB page compression is suitable for a more general case.

  • Columns cannot be indexed, while with InnoDB page compression indexes are possible as usual.

Examples

CREATE TABLE cmp (i TEXT COMPRESSED);

CREATE TABLE cmp2 (i TEXT COMPRESSED=zlib);

See Also

This page is licensed: CC BY-SA / Gnu FDL

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