Re: DISTINCT ON

On Nov 3, 2009, at 10:17 PM, Emmanuel Cecchet <manu(at)asterdata(dot)com>
wrote:

> Hi all,
>
> It looks like Postgres has a restriction in DISTINCT ON queries
> where the DISTINCT ON expressions must match the left side of the
> ORDER BY list. The issue is that if a DISTINCT ON ... has multiple
> instances of a particular expression, this check doesn't seem to
> fire correctly.
>
> For example, this query returns an error (but I guess it shouldn't):
>
> SELECT DISTINCT ON ('1'::varchar, '1'::varchar) a FROM (SELECT 1 AS
> a) AS a ORDER BY '1'::varchar, '1'::varchar, '2'::varchar;
>
> And this query doesn't return an error (but I guess it should):
>
> SELECT DISTINCT ON ('1'::varchar, '2'::varchar, '1'::varchar) a FROM
> (SELECT 1 AS a) AS a ORDER BY '1'::varchar, '2'::varchar,
> '2'::varchar;
>
>
> Am I misunderstanding something or is there a bug?

I'm guessing this is the result of some subtly flakey equivalence
class handling. On first glance ISTM that discarding duplicates is
legit and therefore both examples ought to work...

...Robert