Some of the most important things concerning indexes for the different table types are listed below:

MyISAM
BLOB and TEXT indexes can exist.
Null values are permissible in indexes

MERGE
A MERGE table contains no indexes of its own.

HEAP
Use a hash index stored in memory, which is very fast.
Can only use indexes with the = and <=> operators.
Cannot use an index on a column that allows NULL values.
Indexes are not used with an ORDER BY clause