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.
