| From: | Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> |
|---|---|
| To: | Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> |
| Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Floris Van Nee <florisvannee(at)optiver(dot)com>, "Hou, Zhijie" <houzj(dot)fnst(at)cn(dot)fujitsu(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com> |
| Subject: | Re: UniqueKey on Partitioned table. |
| Date: | 2021-04-06 10:31:02 |
| Message-ID: | CAKU4AWqVcbkH5EnzPT=9hN_xkYZKiRvE2F9Er_qu3ihHoWY7ww@mail.gmail.com |
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On Wed, Mar 31, 2021 at 9:12 PM Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
wrote:
> > b). How to present the element
> > in UniqueKey. Prue EquivalenceClasses or Mix of Expr and
> EquivalenceClass as
> > we just talked about.
> I think the reason we add ECs for sort expressions is to use
> transitive relationship. The EC may start with a single member but
> later in the planning that member might find partners which are all
> equivalent. Result ordered by one is also ordered by the other. The
> same logic applies to UniqueKey as well, isn't it. In a result if a
> set of columns makes a row unique, the set of columns represented by
> the other EC member should be unique. Though a key will start as a
> singleton it might EC partners later and thus thus unique key will
> transition to all the members. With that logic UniqueKey should use
> just ECs instead of bare expressions.
>
TBH, I haven't thought about this too hard, but I think when we build the
UniqueKey, all the ECs have been built already. So can you think out an
case we start with an EC with a single member at the beginning and
have more members later for UniqueKey cases?
--
Best Regards
Andy Fan (https://www.aliyun.com/)
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