(mysql.info) compressed-format

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(mysql.info) dynamic-format

(mysql.info) myisam-table-formats

 
 14.1.3.3 Compressed Table Characteristics
 .........................................
 
 Compressed storage format is a read-only format that is generated with
 the `myisampack' tool.  Compressed tables can be uncompressed with
 `myisamchk'.
 
 Compressed tables have the following characteristics:
 
    * Compressed tables take very little disk space. This minimizes disk
      usage, which is helpful when using slow disks (such as CD-ROMs).
 
    * Each row is compressed separately, so there is very little access
      overhead. The header for a row takes up one to three bytes
      depending on the biggest row 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 data
           type is converted to `ENUM'.
 
         * A column may use any combination of the preceding compression
           types.
 
    * Can be used for fixed-length or dynamic-length rows.
 

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(mysql.info) myisam-table-formats