The MERGE storage engine was introduced in MySQL 3.23.25. It is also
known as the MRG_MyISAM engine.

A MERGE table is a collection of identical MyISAM tables that can be
used as one. "Identical" means that all tables have identical column
and index information. You cannot merge MyISAM tables in which the
columns are listed in a different order, do not have exactly the same
columns, or have the indexes in different order. However, any or all of
the MyISAM tables can be compressed with myisampack. See
http://dev.mysql.com/doc/refman/4.1/en/myisampack.html. Differences in
table options such as AVG_ROW_LENGTH, MAX_ROWS, or PACK_KEYS do not
matter.
