title: Errors when I try to migrate in Django2
body: <p>I have a problem (at least I think). I am new in all this, so I apologize If I ask something stupid. 
  I have some site, which is working normally. When I try to make migrations ( <code>python manage.py makemigrations</code>), everything passed ok, I got the message of how many models new I have, etc. But, when I run after that migrate, I got the following output:</p>
  
  <pre><code>Operations to perform:
    Apply all migrations: admin, auth, comments, contenttypes, news, sessions
    Running migrations:
     Applying comments.0003_auto_20180816_2158...Traceback (most recent call last):
     File "../venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
    psycopg2.ProgrammingError: syntax error at or near "WITH ORDINALITY"
    LINE 6:                     FROM unnest(c.conkey) WITH ORDINALITY co...
                                                ^
  
  
  The above exception was the direct cause of the following exception:
  
  Traceback (most recent call last):
    File "manage.py", line 15, in &lt;module&gt;
     execute_from_command_line(sys.argv)
    File ".../venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
  utility.execute()
   File ".../venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
  self.fetch_command(subcommand).run_from_argv(self.argv)
  File ".../venv/lib/python3.6/site-packages/django/core/management/base.py", line 316, in run_from_argv
  self.execute(*args, **cmd_options)
  File 
    ".../venv/lib/python3.6/site-packages/django/core/management/base.py", line 353, in execute
     output = self.handle(*args, **options)
    File 
  ".../venv/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
       res = handle_func(*args, **kwargs)
     File ".../venv/lib/python3.6/site- 
   packages/django/core/management/commands/migrate.py", line 203, in handle
  fake_initial=fake_initial,
     File 
  
  
   ".../venv/lib/python3.6/site- 
  packages/django/db/backends/base/schema.py", line 531, in _alter_field
  fk_names = self._constraint_names(model, [old_field.column], 
  foreign_key=True)
    File ".../venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 1027, in _constraint_names
  constraints = self.connection.introspection.get_constraints(cursor, model._meta.db_table)
   File ".../venv/lib/python3.6/site- 
  packages/django/db/backends/postgresql/introspection.py", line 158, in get_constraints
  """, ["public", table_name])
   File ".../venv/lib/python3.6/site-packages/django/db/backends/utils.py", 
   line 100, in execute
     return super().execute(sql, params)
   File 
  
  
  ".../venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
     return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
    File ".../venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
  return executor(sql, params, many, context)
    File ".../venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
  return self.cursor.execute(sql, params)
    File "/.../venv/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
  raise dj_exc_value.with_traceback(traceback) from exc_value
    File ".../venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
   django.db.utils.ProgrammingError: syntax error at or near "WITH 
  ORDINALITY"
  LINE 6:                     FROM unnest(c.conkey) WITH ORDINALITY co...
                                                ^
  </code></pre>
  
  <p>Anyway, if after that I try again to make migrations,  got the message that I don't have migrations. So, Django did the job, But this error is here constantly when I try to migrate, I am really wondering why. I tried to google it, but I got nothing. </p>