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...
Thanks for clarifying that. The relevant section in the release notes (which I managed to miss) is this: