| From: | Peter Eisentraut <e99re41(at)DoCS(dot)UU(dot)SE> |
|---|---|
| To: | Zeugswetter Andreas SB <ZeugswetterA(at)wien(dot)spardat(dot)at> |
| Cc: | "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "'Jose Soares'" <jose(at)sferacarta(dot)com>, "'hackers'" <pgsql-hackers(at)postgreSQL(dot)org> |
| Subject: | Re: AW: [HACKERS] having and union in v7beta |
| Date: | 2000-03-02 16:23:35 |
| Message-ID: | Pine.GSO.4.02A.10003021718390.27493-100000@Dront.DoCS.UU.SE |
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| Lists: | pgsql-hackers |
On Thu, 2 Mar 2000, Zeugswetter Andreas SB wrote:
> But if the two queries are the same, the union CAN be simplified,
> since the union of two identical masses (I don't know the correct word here)
> is still that one mass.
"set" :)
>
> Thus 6.5 simplification is correct in this particular case.
The issue here seems to be that the queries could have side-effects, such
as
select nextval('sequence1')
union
select nextval('sequence1')
which should arguably return two distinct rows. I gotta reread SQL's
opinion on this, but I'm sure Tom has already done that. From a
mathematical point of view, I believe your assumption "lexically equal
queries yield mathematically equal sets" is wrong.
--
Peter Eisentraut Sernanders väg 10:115
peter_e(at)gmx(dot)net 75262 Uppsala
http://yi.org/peter-e/ Sweden
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