Re: BUG #17150: Unexpected outputs from the query

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Tuesday, August 17, 2021, PG Bug reporting form <noreply@postgresql.org>
> wrote:
>> This
>> unexpected return can be fixed by removing "ORDER BY ( SELECT COUNT ( v1 )
>> )", then the query returns sum="0" as expected.

> Well, PostgreSQL cannot remove the order by otherwise it would be a
> different query.  So your suggestion is spot on, and the user should
> probably do that, but it doesn’t seem like a bug.

Yeah.  PG interprets

    SELECT x FROM v2 ORDER BY (SELECT COUNT(v1))

to behave the same as

    SELECT x, (SELECT COUNT(v1)) FROM v2 ORDER BY 2

(modulo the fact that the ORDER BY column won't be output),
and then it turns out that that's effectively the same as

    SELECT x, COUNT(v1) FROM v2 ORDER BY 2

the reason being that since v1 is a variable of the outer query,
the aggregate is considered to be an aggregate of the outer query
*not* the sub-select.  (That's required by the SQL standard.)
So at this point you have an aggregated query that is certain
to return 1 row, not more or less, regardless of how many rows
are returned by v2.

            regards, tom lane