equal: don't know whether nodes of type 719 are equal

I'm getting this message in my log files. It's a warning being generated
when I submit a specific query to Postgres. Here's the Perl/DBI code that
submits the query:

  $sth = $self->{dbh}->prepare(
    "select distinct event.* from \
    event, u_participant, group_user, g_participant where \
    event.t_begin >= '$date 0:00' and \
    event.t_end <= '$date 23:59' and \
    (g_participant.event_id = event.event_id and \
     g_participant.group_id = group_user.group_id and \
     group_user.username = varchar '$username') \
    union
    select distinct event.* from \
    event, u_participant, group_user, g_participant where \
    event.t_begin >= '$date 0:00' and \
    event.t_end <= '$date 23:59' and \
    (u_participant.event_id = event.event_id and \
     u_participant.username = varchar '$username')");
  $rv = $sth->execute;

event_id and group_id are all int4.

username is varchar(10).

t_begin and t_end are datetime.

I know the message is ONLY a warning... but I'd like to know what it's
warning me about.

-=Fozz

--
Doran L. Barton <fozz@iodynamics.com>
Iodynamics LLC -- "Internetworking the masses"
<URL:http://www.iodynamics.com/>