Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> Ian Barwick wrote:
>> Note I'm not sure whether this is a bug, or whether the assumption
>> made for the original query (that the row order returned by the
>> subquery would be carried over to the main part of the query) is
>> incorrect but just happened to work as expected pre-8.4.
> The latter. Without an ORDER BY (at the outermost level), the order of
> the result is not well defined. Before 8.4, UNION was always performed
> by a Sort + Unique, which explains why the output is always sorted in
> previous releases. 8.4 knows how to perform it with a Hash Aggregate,
> which doesn't yield sorted output.
This is mentioned in the release notes, but I suppose we'd better
promote it to the "observe the following incompatibilities" list...
regards, tom lane