From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Mark Rofail <markm.rofail@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>
Cc: David Steele <david@pgmasters.net>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: pgsql-hackers-owner@postgresql.org, Erik Rijkers <er@xs4all.nl>
Subject: Re: GSoC 2017: Foreign Key Arrays
Date: Mon, 31 Jul 2017 11:18:05 -0400
Message-ID: <20170731151805.nonu6qwv54ca4qvv@alvherre.pgsql> (raw)
In-Reply-To: <17185.1501392410@sss.pgh.pa.us>
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Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > ... However, when you create an index, you can
> > indicate which operator class to use, and it may not be the default one.
> > If a different one is chosen at index creation time, then a query using
> > COUNT(distinct) will do the wrong thing, because DISTINCT will select
> > an equality type using the type's default operator class, not the
> > equality that belongs to the operator class used to create the index.
>
> > That's wrong: DISTINCT should use the equality operator that corresponds
> > to the index' operator class instead, not the default one.
>
> Uh, what? Surely the semantics of count(distinct x) *must not* vary
> depending on what indexes happen to be available.
Err ...
> I think what you meant to say is that the planner may only choose an
> optimization of this sort when the index's opclass matches the one
> DISTINCT will use, ie the default for the data type.
Um, yeah, absolutely.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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Cc: alvherre@2ndquadrant.com, tgl@sss.pgh.pa.us, markm.rofail@gmail.com, robertmhaas@gmail.com, aekorotkov@gmail.com, david@pgmasters.net, sfrost@snowman.net, er@xs4all.nl
Subject: Re: GSoC 2017: Foreign Key Arrays
In-Reply-To: <20170731151805.nonu6qwv54ca4qvv@alvherre.pgsql>
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