| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
| Cc: | John Lumby <johnlumby(at)hotmail(dot)com>, "pgsql-docs(at)lists(dot)postgresql(dot)org" <pgsql-docs(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: description of Aggregate Expressions |
| Date: | 2019-12-05 23:29:34 |
| Message-ID: | 25458.1575588574@sss.pgh.pa.us |
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| Lists: | pgsql-admin pgsql-docs |
"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Thu, Dec 5, 2019 at 3:18 PM John Lumby <johnlumby(at)hotmail(dot)com> wrote:
>> 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.
Yeah. This might be more intelligible if it were written
select count(DISTINCT ROW(parent_id, name) ) from mytable
However, the SQL committee in their finite wisdom have decreed that
the ROW keyword is optional. (As long as there's more than one
column expression; the need for that special case is another reason
why omitting ROW isn't really a nice thing to do.)
regards, tom lane
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