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    1. General Information
    2. MySQL Installation
    3. Tutorial Introduction
    4. Database Administration
    5. MySQL Optimisation
    6. MySQL Language Reference
    7. MySQL Table Types
    8. MySQL APIs
    9. Extending MySQL
    Chapter 7:  MySQL Table Types 499   If  you  update  a  row  with  information  that  extends  the  row  length,  the  row  will  be fragmented.  In this case, you may have to run myisamchk -r from time to time to get better performance.  Use myisamchk -ei tbl_name for some statistics.   Not as easy to reconstruct after a crash, because a record may be fragmented into many pieces and a link (fragment) may be missing.   The expected row length for dynamic sized records is: 3 + (number of columns + 7) / 8 + (number of char columns) + packed size of numeric columns + length of strings + (number of NULL columns + 7) / 8 There is a penalty of 6 bytes for each link.  A dynamic record is linked whenever an update causes an enlargement of the record.  Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link.  If not, there will be another link.  You may check how many links there are with myisamchk -ed.  All links may be removed with myisamchk -r. 7.1.2.3  Compressed Table Characteristics 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 minimises 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 xed (1-3 bytes) depending on the biggest record in the table.  Each column is compressed di erently.  Some of the compression types are: There is usually a di erent Hu man table for each column. Sux space compression. Pre x 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 xed- or dynamic-length records.    Can be uncompressed with myisamchk.
     

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