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Cc: "Pavel Stehule" <pavel.stehule@gmail.com>, "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
From: "Dian M Fay" <dian.m.fay@gmail.com>
To: "Dmitry Dolgov" <9erthalion6@gmail.com>
Date: Wed, 20 Jan 2021 23:37:32 -0500
Message-Id: <C8OK3JQNE1KN.2H0O0RCBD576E@lamia>
In-Reply-To: <20210120190857.vvtwtabubwxzeraq@localhost>
Precedence: bulk

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On Wed Jan 20, 2021 at 2:08 PM EST, Dmitry Dolgov wrote:
> > On Wed, Jan 20, 2021 at 11:34:16AM -0500, Dian M Fay wrote:
> > > Thanks, I need to remember to not skipp doc building for testing proc=
ess
> > > even for such small changes. Hope now I didn't forget anything.
> > >
> > > > On Wed, Jan 20, 2021 at 09:58:43AM -0500, Dian M Fay wrote:
> > >
> > > > Here's a full editing pass on the documentation, with v45 and Pavel=
's
> > > > doc-whitespaces-fix.patch applied. I also corrected a typo in one o=
f the
> > > > added hints.
> > >
> > > Great! I've applied almost all of it, except:
> > >
> > > + A <type>jsonb</type> value will accept assignments to nonexistent
> > > subscript
> > > + paths as long as the nonexistent elements being traversed are all
> > > arrays.
> > >
> > > Maybe I've misunderstood the intention, but there is no requirement
> > > about arrays for creating such an empty path. I've formulated it as:
> > >
> > > + A <type>jsonb</type> value will accept assignments to nonexistent
> > > subscript
> > > + paths as long as the last existing path key is an object or an arra=
y.
> >
> > My intention there was to highlight the difference between:
> >
> > * SET obj['a']['b']['c'] =3D '"newvalue"'
> > * SET arr[0][0][3] =3D '"newvalue"'
> >
> > obj has to conform to {"a": {"b": {...}}} in order to receive the
> > assignment of the nested c. If it doesn't, that's the error case we
> > discussed earlier. But arr can be null, [], and so on, and any missing
> > structure [[[null, null, null, "newvalue"]]] will be created.
>
> If arr is 'null', or any other scalar value, such subscripting will work
> only one level deep because they represented internally as an array of
> one element. If arr is '[]' the path will comply by definition. So it's
> essentially the same as for objects with no particular difference. If
> such a quirk about scalars being treated like arrays is bothering, we
> could also bend it in this case as well (see the attached version).

I missed that distinction in the original UPDATE paragraph too. Here's
another revision based on v48.

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