Date: Wed, 3 Jul 2019 21:46:28 +0200
From: David Fetter <david@fetter.org>
To: David Rowley <david.rowley@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@gmail.com>,
	Jesper Pedersen <jesper.pedersen@redhat.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Floris Van Nee <florisvannee@optiver.com>,
	James Coleman <jtc331@gmail.com>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Jeff Janes <jeff.janes@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Thomas Munro <thomas.munro@enterprisedb.com>,
	Bhushan Uparkar <bhushan.uparkar@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>
Subject: Re: Index Skip Scan
Message-ID: <20190703194627.GA24679@fetter.org>
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On Wed, Jul 03, 2019 at 12:27:09AM +1200, David Rowley wrote:
> On Tue, 2 Jul 2019 at 21:00, Thomas Munro <thomas.munro@gmail.com> wrote:
> 
> The more I think about these UniqueKeys, the more I think they need to
> be a separate concept to PathKeys. For example, UniqueKeys: { x, y }
> should be equivalent to { y, x }, but with PathKeys, that's not the
> case, since the order of each key matters. UniqueKeys equivalent
> version of pathkeys_contained_in() would not care about the order of
> individual keys, it would say things like, { a, b, c } is contained in
> { b, a }, since if the path is unique on columns { b, a } then it must
> also be unique on { a, b, c }.

Is that actually true, though? I can see unique {a, b, c} => unique
{a, b}, but for example:

a | b | c
--|---|--
1 | 2 | 3
1 | 2 | 4
1 | 2 | 5

is unique on {a, b, c} but not on {a, b}, at least as I understand the
way "unique" is used here, which is 3 distinct {a, b, c}, but only one
{a, b}.

Or I could be missing something obvious, and in that case, please
ignore.

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

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