On Thu, Dec 5, 2019 at 3:18 PM John Lumby <johnlu...@hotmail.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.
