Date: Fri, 11 Dec 2020 16:19:57 +0100
From: Dmitry Dolgov <9erthalion6@gmail.com>
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
	Alexander Korotkov <aekorotkov@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	David Steele <david@pgmasters.net>,
	Nikita Glukhov <n.gluhov@postgrespro.ru>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	David Fetter <david@fetter.org>,
	Thomas Munro <thomas.munro@gmail.com>,
	Oleksandr Shulgin <oleksandr.shulgin@zalando.de>,
	Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael.paquier@gmail.com>,
	Oleg Bartunov <obartunov@gmail.com>,
	Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] [PATCH] Generic type subscripting
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> On Wed, Dec 09, 2020 at 04:59:34PM -0500, Tom Lane wrote:
>
> 0001 adds the ability to attach a subscript handler to an existing
> data type with ALTER TYPE.  This is clearly going to be necessary
> if we want extension types to be able to use this facility.  The
> only thing that I think might be controversial here is that I did
> not add the ability to set pg_type.typelem.  While that'd be easy
> enough so far as ALTER TYPE is concerned, I'm not sure that we want
> to encourage people to change it.  The dependency rules mean that
> the semantics of typelem aren't something you really want to change
> after-the-fact on an existing type.  Also, if we did allow it, any
> existing SubscriptingRef.refelemtype values in stored views would
> fail to be updated.

I'm curious what could be the use case for setting pg_type.typelem for
subscripting? I don't see this that much controversial, but maybe I'm
missing something.

> On Thu, Dec 10, 2020 at 05:37:20AM +0100, Pavel Stehule wrote:
> st 9. 12. 2020 v 22:59 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:
>
> > 0002 makes use of that to support subscripting of hstore.  I'm not
> > sure how much we care about that from a functionality standpoint,
> > but it seems like it might be good to have a contrib module testing
> > that extensions can use this.  Also, I thought possibly an example
> > showing what's basically the minimum possible amount of complexity
> > would be good to have.  If people like this, I'll finish it up (it
> > lacks docs) and add it.
> >
>
> +1 using subscripts for hstore is nice idea

Yeah, I also find it's a good suggestion, the implementation seems fine
as well. As a side note, I'm surprised hstore doesn't have any
functionality to update values, except hstore_concat.