Chris <chris@bitmead.com> writes:
>> select distinct x from foo order by y;
>>
>> which produces ill-defined results.
> Why is this ill-defined? If y is in x then it is also distinct
Huh? The query specifies distinct values of x, and only x.
Consider x y
1 1 1 10 2 0 2 11
"select distinct x" ought to produce one row with x=1, and one row with
x=2, and nothing else. If it implicitly did the distinct on y as well,
you'd get four rows with two x=1 and two x=2, which is not my idea of
"distinct x". But if you don't have four rows out, then there's no
meaningful way to order by y.
6.5.3 in fact produces four rows from this query, which is generally
conceded to be broken behavior.
regards, tom lane