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) );