What is a federated index of MySQL

CREATE TABLE ' Test ' (' AAA ' varchar (+) not NULL default ',
' BBB ' varchar (+) NOT null default ' ', ' CCC ' int (one) UNSIGNED NOT null default 0,
KEY ' Sindex ' (' AAA ', ' BBB ', ' CCC ') ') Engine=myisam comment= ';
This establishes a joint index on the AAA, BBB, CCC 3 columns.
If the table is already built, do it in phpMyAdmin:
Alert table Test Add INDEX ' sindex ' (' AAA ', ' BBB ', ' CCC ')
You can create a federated index on these 3 columns.

What is a federated index of MySQL