python inspectdb got "The error was: function unnest(smallint[]) does not exist"

I use Amazon Redshift database and Django 2.0.6.   My redshift database settings is as below:

DATABASES = {

    'default': {

        'NAME': 'my_db_name',

        'ENGINE': 'django.db.backends.postgresql_psycopg2',

        'USER': 'my_username',

        'PASSWORD': 'my_password',

        'HOST': 'my_hostname',

        'PORT': 5439,

    },

}

When I run "python3 manage.py inspectdb", I got below error messages:

# Unable to inspect table 'eld_messages'

# The error was: function unnest(smallint[]) does not exist

HINT:  No function matches the given name and argument types. You may need to add explicit type casts.

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