Re: [HACKERS] Re: SELECT DISTINCT ON ... ORDER BY ...

Tom Lane wrote:
>Thomas Metz <tmetz(at)gsf(dot)de> writes:
>> SELECT DISTINCT ON id id, name FROM test ORDER BY name;
>> [doesn't work as expected]
>
>There have been related discussions before on pg-hackers mail list;
>you might care to check the list archives. The conclusion I recall
>is that it's not real clear how the combination of SELECT DISTINCT
>on one column and ORDER BY on another *should* work. Postgres'
>current behavior is clearly wrong IMHO, but there isn't a unique
>definition of right behavior, because it's not clear which tuples
>should get selected for the sort.
>
>This "SELECT DISTINCT ON attribute" option strikes me as even more
>bogus. Where did we get that from --- is it in the SQL92 standard?

I looked through the standard yesterday and couldn't find it. It doesn't
seem to be a useful extension, since it does nothing that you can't do
with GROUP BY and seems much less well defined. For the moment I have
added a brief description to the reference documentation for SELECT.

>If you SELECT DISTINCT on a subset of the attributes to be returned,
>then there's no unique definition of which values get returned in the
>other columns. In Thomas' example:
...
>Any of these are "DISTINCT ON id", but it's purely a matter of
>happenstance table order and unspecified implementation choices which
>one will appear. Do we really have (or want) a statement with
>inherently undefined behavior?

We have it; I suggest we don't want it!

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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