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(mysql.info)Compressed format

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Compressed Table Characteristics
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This is a read-only type that is generated with the optional
`myisampack' tool (`pack_isam' for `ISAM' tables):

   * All MySQL distributions, even those that existed before MySQL went
     GPL, can read tables that were compressed with `myisampack'.

   * Compressed tables take very little disk space. This minimizes disk
     usage, which is very nice when using slow disks (like CD-ROMs).

   * Each record is compressed separately (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. Some of the compression types are:
        - There is usually a different Huffman table for each column.

        - Suffix space compression.

        - Prefix space compression.

        - Numbers with value `0' are stored using 1 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 (8 bytes) may be stored as a `TINYINT' column
          (1 byte) if all values are in the range `0' to `255'.

        - 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 above compressions.

   * Can handle fixed- or dynamic-length records, but not `BLOB' or
     `TEXT' columns.

   * Can be uncompressed with `myisamchk'.