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
select count(DISTINCT parent_id , name) from mytable
but that is rejected with
ERROR: function count(bigint, text) does not exist
whereas
select count(DISTINCT ( parent_id , name) ) from mytable
is accepted.
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.
Cheers, John Lumby