>> 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. [1] https://www.postgresql.org/message-id/26825.1064858...@sss.pgh.pa.us [2] https://www.postgresql.org/docs/current/queries-union.html
