Re: BUG #1171: select distinct 'value',... fails.

"PostgreSQL Bugs List" <pgsql-bugs@postgresql.org> writes:
>     select distinct 'value';
> ERROR:  Unable to identify an ordering operator '<' for type '"unknown"'
>         Use an explicit ordering operator or modify the query

This is accepted in 7.4 and later.  You are evidently using something
older.

            regards, tom lane