(mysql.info.gz) Compressed format

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 14.1.3.3 Compressed Table Characteristics
 .........................................
 
 Compressed storage format is a read-only format that is generated with
 the `myisampack' tool.
 
 All MySQL distributions as of version 3.23.19 include `myisampack' by
 default. (This version is when MySQL was placed under the GPL.) For
 earlier versions, `myisampack' was included only with licenses or
 support agreements, but the server still can read tables that were
 compressed with `myisampack'.  Compressed tables can be uncompressed
 with `myisamchk'.  (For the `ISAM' storage engine, compressed tables
 can be created with `pack_isam' and uncompressed with `isamchk'.)
 
 Compressed tables have the following characteristics:
 
    * Compressed tables take very little disk space. This minimizes disk
      usage, which is very nice when using slow disks (such as CD-ROMs).
 
    * Each record is compressed separately, so there is very little
      access overhead.  The header for a record is fixed (1-3 bytes)
      depending on the biggest record in the table.  Each column is
      compressed differently.  There is usually a different Huffman tree
      for each column.  Some of the compression types are:
 
         - Suffix space compression.
 
         - Prefix space compression.
 
         - Numbers with a value of zero are stored using one bit.
 
         - If values in an integer column have a small range, the column
           is stored using the smallest possible type. For example, a
           `BIGINT' column (eight bytes) can be stored as a `TINYINT'
           column (one byte) if all its values are in the range from
           `-128' to `127'.
 
         - If a column has only a small set of possible values, the
           column type is converted to `ENUM'.
 
         - A column may use a combination of the preceding compressions.
 
 
    * Can handle fixed-length or dynamic-length records.
 
 

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