create table users ( id int not null primary key auto_increment, nick varchar(150));create index idx_users on users ( nick(150) );create table sessions ( id int not null primary key auto_increment, idFrom int not null, idTo int not null, constraint fk_session_user_from foreign key (idFrom) references user(id), constraint fk_session_user_to foreign key (idTo) references user(id));create index idx_session on sessions ( idFrom , idTo );create table messages ( id int not null primary key auto_increment, idSession int not null, message varchar(4000), mtime timestamp not null default current_timestamp, constraint fk_message_session_id foreign key (idSession) references session(id));create index idx_messages on messages ( idSession , message(500) );