message_id          |    text
-------------------------------
        1           |    blablabla
-------------------------------
        2           |   ololo
-------------------------------
        3           |   troololo

user_id             |    message_id
-------------------------------
        1           |    1
-------------------------------
        1           |   2
-------------------------------
        2           |   1
-------------------------------
        2           |   3

select text from A join B using (message_id) where chat_id = 1 and user_id = 2 order by message_id;

create table chat_flags(
  flag bigint,
  description character varying (256),
  constraint chat_flags_pk primary key (flag)
);

create table chats(
  chat_id bigint,
  title character varying (256),
  avatar character varying (256),
  flags bigint,
  constraint chats_pk primary key (chat_id)
);

create table users(
  user_id bigint,
  nickname character varying (256),
  avatar character varying (256),
  constraint users_pk primary key (user_id)
);

create table messages(
  message_id bigint,
  user_id bigint,
  message text,
  creation_time timestamp without time zone,
  constraint messages_pk primary key (message_id),
  constraint messages_fk foreign key (user_id)
    references users (user_id)
);

create table chat_user_messages(
  chat_id bigint,
  user_id bigint,
  message_id bigint,
  constraint chat_user_messages_pk (chat_id, user_id, message_id),
  constraint chat_user_messages_fk1 foreign key (chat_id)
    references chats (chat_id)
  constraint chat_user_messages_fk2 foreign key (user_id)
    references users (user_id),
  constraint chat_user_messages_fk3 foreign key (message_id)
    references chats (message_id)
);