Re: description of Aggregate Expressions

On Thu, Dec 5, 2019 at 3:18 PM John Lumby <johnlumby(at)hotmail(dot)com> wrote:

> In PostgreSQL 12.1 Documentation chapter 4.2.7. Aggregate Expressions it
> says
>
>
> The syntax of an aggregate expression is one of the following:
> ...
> aggregate_name (DISTINCT expression [ , ... ] [ order_by_clause ] ) [
> FILTER ( WHERE filter_clause ) ]
> ...
>
> I believe this is incorrect in the case where the DISTINCT is on a
> comma-separated list of expressions.
> It would imply that this is legal
>

It is...you didn't get a syntax error.

>
> select count(DISTINCT parent_id , name) from mytable
>
> but that is rejected with
> ERROR: function count(bigint, text) does not exist
>

The error is that while the query is syntactically correct in order to
execute it as written a function would need to exist that does not. As far
as a general syntax diagram goes it has correctly communicated what is
legal.

> whereas
>
> select count(DISTINCT ( parent_id , name) ) from mytable
>
> is accepted.
>

Correct, converting the two individual columns into a "tuple" allows the
default tuple distinct-making infrastructure to be used to execute the
query.

> So I think to handle all cases the line in the doc should read
>
> aggregate_name (DISTINCT ( expression [ , ... ] ) [ order_by_clause ] ) [
> FILTER ( WHERE filter_clause ) ]
>
> I don't know how to indicate that those extra parentheses can be omitted
> if the list has only one expression.
>

Then I would have to say the proposed solution to this edge case is worse
than the problem. I also don't expect there to be a clean solution to
dealing with the complexities of expressions at the syntax diagram level.

David J.