Re: Clarify the ordering guarantees in combining queries (or lack thereof)

On Thu, Jul 14, 2022 at 9:16 AM Shay Rojansky <roji(at)roji(dot)org> wrote:

>
> >> I was trying to understand what - if any - are the guarantees with
> regards to ordering for combining queries (UNION/UNION ALL/...). From this
> message[1], it seems that UNION ALL does preserve the ordering of the
> operand queries, whereas UNION does not (presumably neither do INTERSECT,
> INTERSECT ALL, EXCEPT and EXCEPT ALL).
> >>
> >> The documentation[2] makes no mention of this, I'd suggest adding a
> note clarifying this.
> >
> > If you want ordered output use ORDER BY.
>
> I don't see how that could be done. Consider the following:
>
> (SELECT id FROM data ORDER BY id)
> UNION ALL
> (SELECT id FROM data ORDER BY id DESC);
>
> If there's a guarantee that UNION ALL preserves ordering - as Tom seems to
> indicate in the thread quoted above - then the above works. If there's no
> such guarantee, then AFAIK the above can't be rewritten; putting the ORDER
> BY outside - on the results of the UNION ALL - would order all results
> rather than preserving each resultset's ordering.
>
> No, there is no guarantee. It's just that UNION ALL works this way today
(preserving the order of the subselects) - and I'm not even sure about
that, it may not preserve the order in all cases, with different indexes or
partitioning or a parallel plan, etc.
In any case, there is no guarantee that the behaviour will not change in
the future due to planner improvements.

Best regards
Pantelis Theodosiou

> [1] https://www.postgresql.org/message-id/26825.1064858799@sss.pgh.pa.us
> [2] https://www.postgresql.org/docs/current/queries-union.html
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