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(mysql.info)Design

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Design Choices
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MySQL keeps row data and index data in separate files. Many (almost
all) other databases 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 will
cause 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
(like in Oracle/Sybase et al). In this case you will 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 can't use only the index table to retrieve data for a query.

   * You lose a lot of space, as you must duplicate indexes from the
     nodes (as you can't store the row in the nodes).

   * Deletes will degenerate the table over time (as indexes in nodes
     are usually not updated on delete).

   * It's harder to cache ONLY the index data.