Created a separate table to store the aliases of an user:

CREATE TABLE user_alias (
cid INT4 UNSIGNED NOT NULL,
user INT4 UNSIGNED NOT NULL,
alias VARCHAR(255) NOT NULL,
PRIMARY KEY (cid, user, alias)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

For compatibility reason the aliases will be also stored in the user_attributes table.

A blocking update task will be executed which creates the new user_alias table, furthermore the aliases from the user_attribute table will be inserted in the user_alias table. For compatibility reason the aliases are not deleted in the user_attribute table.