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help / color / mirror / Atom feedFrom: David G. Johnston <[email protected]>
To: John Lumby <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: description of Aggregate Expressions
Date: Thu, 5 Dec 2019 16:06:12 -0700
Message-ID: <CAKFQuwZk3Kt4uNktMgLYj5B1Rzrm1R4EMVZQRka189AhnPkM=Q@mail.gmail.com> (raw)
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On Thu, Dec 5, 2019 at 3:18 PM John Lumby <[email protected]> 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.
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To: [email protected]
Cc: [email protected], [email protected], [email protected]
Subject: Re: description of Aggregate Expressions
In-Reply-To: <CAKFQuwZk3Kt4uNktMgLYj5B1Rzrm1R4EMVZQRka189AhnPkM=Q@mail.gmail.com>
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