Ce inseamna KEY la creare tabel in MySQL

[patricia]

Ce inseamna cand creezi un tabel, cand in dreptul unei coloane pui cuvantul KEY, daca PRIMARY KEY identifica in mod unic fiecare rand din tabel, KEY ce reprezinta?

Cod: Selectaţi tot

CREATE TABLE news (
	id int(11) NOT NULL AUTO_INCREMENT,
	title varchar(128) NOT NULL,
	slug varchar(128) NOT NULL,
	text text NOT NULL,
	PRIMARY KEY (id),
	KEY slug (slug)
);

Din ce am citit pe net, KEY e un sinonim pentru INDEX, dar KEY ar fi deprecat /nefolosit in MySQL 5.5+.
In documentatia MySQL de la adresa:
dev.mysql.com/doc/refman/5.1/en/create-table.html
Se specifica:

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.
...
index_option values specify additional options for an index. USING is one such option.