Indexing for High Performance - High Performance MySQL - page 172

What about the index on col2 ? Is there anything special about it? As it turns out, no—
it's just an index like any other. Figure 5-6 illustrates the col2 index.

Figure 5-6. MyISAM col2 index layout for the layout_test table

In fact, in MyISAM, there is no structural difference between a primary key and any
other index. A primary key is simply a unique, nonnullable index named PRIMARY .
InnoDB stores the same data very differently because of its clustered
organization. InnoDB stores the table as shown in Figure 5-7 .

InnoDB's data layout.

Figure 5-7. InnoDB primary key layout for the layout_test table

At first glance, that might not look very different from Figure 5-5 . But look again, and
notice that this illustration shows the whole table , not just the index. Because the
clustered index “is” the table in InnoDB, there's no separate row storage as there is for
MyISAM.