(mysql.info) design

(mysql.info) optimizing-database-structure

 
 7.4.1 Design Choices
 --------------------
 
 MySQL keeps row data and index data in separate files. Many (almost
 all) other database systems mix row and index data in the same file. We
 believe that the MySQL choice is better for a very wide range of modern
 systems.
 
 Another way to store the row data is to keep the information for each
 column in a separate area (examples are SDBM and Focus).  This causes a
 performance hit for every query that accesses more than one column.
 Because this degenerates so quickly when more than one column is
 accessed, we believe that this model is not good for general-purpose
 databases.
 
 The more common case is that the index and data are stored together (as
 in Oracle/Sybase, et al). In this case, you find the row information at
 the leaf page of the index. The good thing with this layout is that it,
 in many cases, depending on how well the index is cached, saves a disk
 read. The bad things with this layout are:
 
    * Table scanning is much slower because you have to read through the
      indexes to get at the data.
 
    * You cannot use only the index table to retrieve data for a query.
 
    * You use more space because you must duplicate indexes from the
      nodes (you cannot store the row in the nodes).
 
    * Deletes degenerate the table over time (because indexes in nodes
      are usually not updated on delete).
 
    * It is more difficult to cache only the index data.

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