text: create table groups_member
  (
      gid         bigint    not null,
      uid         bigint    not null,
      create_time timestamp not null
  );
  
  comment on table groups_member is '群成员';
  
  comment on column groups_member.gid is '群 ID';
  
  comment on column groups_member.uid is '用户 ID';
  
  comment on column groups_member.create_time is '创建时间';
  
  create unique index groups_member_gid_uid_uindex
      on groups_member (gid, uid);