Re: problem with subselect

Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> "Sub-SELECT uses un-GROUPed attribute f.date_creation from outer query"
>> Which is not really true, but some way is...

> Hmmm ... this looks like a minor parser bug.

No, it's operating as designed. Note the comments in CVS tip
parse_agg.c:

* NOTE: we recognize grouping expressions in the main query, but only
* grouping Vars in subqueries. For example, this will be rejected,
* although it could be allowed:
* SELECT
* (SELECT x FROM bar where y = (foo.a + foo.b))
* FROM foo
* GROUP BY a + b;
* The difficulty is the need to account for different sublevels_up.
* This appears to require a whole custom version of equal(), which is
* way more pain than the feature seems worth.

Previous versions implemented the check for ungrouped vars a little
differently, but the net effect was the same. (Given that SQL99 allows
only a simple column reference as a GROUP BY element, this isn't a spec
violation, merely a limitation on how far we are willing to extend the
spec.)

There are a number of straightforward ways to rewrite the query to avoid
this, but I can't help wondering whether the basic approach isn't wrong.
The subselect seems an ugly and inefficient way to do it, because it's
re-executing the entire join for each group ... but I can't quite put
my finger on a better way ...

regards, tom lane