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From: David Fetter <david@fetter.org>
To: David Rowley <david.rowley@2ndquadrant.com>
Cc: James Coleman <jtc331@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Floris Van Nee <florisvannee@optiver.com>
Cc: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: Jeff Janes <jeff.janes@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Bhushan Uparkar <bhushan.uparkar@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Subject: Re: Index Skip Scan
Date: Thu, 4 Jul 2019 02:41:18 +0200
Message-ID: <20190704004118.GB24679@fetter.org> (raw)
In-Reply-To: <CAKJS1f_0aUV6SSKTMFjm2QyT8LiDedJEAZtrAEEi-qQHxRXgng@mail.gmail.com>
References: <4d469bf2-49ff-bc3d-a428-6c7b5d87d526@redhat.com>
	<CAKJS1f86FgODuUnHiQ25RKeuES4qTqeNxm1QbqJWrBoZxVGLiQ@mail.gmail.com>
	<CA+q6zcXoYjCtyU-jFU_=p2DL_K3BHbBEEbJHaJTxdvRX3cU-Sg@mail.gmail.com>
	<CA+q6zcVVigO_1KRBxvzKfnR593CEE7FKR-bwP-XcH6AVHOo4+g@mail.gmail.com>
	<a8b5c2d9-7b23-e12b-fc6b-eee63dc8dacf@redhat.com>
	<CA+hUKGKo30N5VNuRWhDuMGVZ3hTcv4J5RGXe286GRJZLk_jBYQ@mail.gmail.com>
	<CAKJS1f_v5X1Sb42ShOhsLzuMA6iX52PcBwpf4P-L9H_GDYA3-Q@mail.gmail.com>
	<20190703194627.GA24679@fetter.org>
	<CAAaqYe_Yg6uYr-2PG9emw9Nh8ojA0wuDEOVWUBqDaHYpJ7gNgA@mail.gmail.com>
	<CAKJS1f_0aUV6SSKTMFjm2QyT8LiDedJEAZtrAEEi-qQHxRXgng@mail.gmail.com>

On Thu, Jul 04, 2019 at 10:06:11AM +1200, David Rowley wrote:
> On Thu, 4 Jul 2019 at 09:02, James Coleman <jtc331@gmail.com> wrote:
> > I think that example is the opposite direction of what David (Rowley)
> > is saying. Unique on {a, b} implies unique on {a, b, c} while you're
> > correct that the inverse doesn't hold.
> >
> > Unique on {a, b} also implies unique on {b, a} as well as on {b, a, c}
> > and {c, a, b} and {c, b, a} and {a, c, b}, which is what makes this
> > different from pathkeys.
> 
> Yeah, exactly. A superset of the unique columns is still unique.

Thanks for clarifying!

Best,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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  Cc: david@fetter.org, david.rowley@2ndquadrant.com, jtc331@gmail.com, thomas.munro@gmail.com, jesper.pedersen@redhat.com, 9erthalion6@gmail.com, florisvannee@optiver.com, rafia.pghackers@gmail.com, jeff.janes@gmail.com, pg@bowt.ie, tomas.vondra@2ndquadrant.com, thomas.munro@enterprisedb.com, bhushan.uparkar@gmail.com, a.korotkov@postgrespro.ru
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