Re: [HACKERS] [PATCH] Generic type subscripting

From: "Dian M Fay" <dian(dot)m(dot)fay(at)gmail(dot)com>
To: "Dmitry Dolgov" <9erthalion6(at)gmail(dot)com>
Cc: "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Andres Freund" <andres(at)anarazel(dot)de>, "Alexander Korotkov" <aekorotkov(at)gmail(dot)com>, "Justin Pryzby" <pryzby(at)telsasoft(dot)com>, "David Steele" <david(at)pgmasters(dot)net>, "Nikita Glukhov" <n(dot)gluhov(at)postgrespro(dot)ru>, "Alvaro Herrera" <alvherre(at)2ndquadrant(dot)com>, "David Fetter" <david(at)fetter(dot)org>, "Thomas Munro" <thomas(dot)munro(at)gmail(dot)com>, "Oleksandr Shulgin" <oleksandr(dot)shulgin(at)zalando(dot)de>, "Robert Haas" <robertmhaas(at)gmail(dot)com>, "Michael Paquier" <michael(dot)paquier(at)gmail(dot)com>, "Oleg Bartunov" <obartunov(at)gmail(dot)com>, "Peter Eisentraut" <peter(dot)eisentraut(at)2ndquadrant(dot)com>, "PostgreSQL Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] [PATCH] Generic type subscripting
Date: 2021-01-20 16:34:16
Message-ID: C8O4PRM04CWJ.204R7D05JOJLB@lamia
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On Wed Jan 20, 2021 at 11:22 AM EST, Dmitry Dolgov wrote:
> > On Tue Jan 19, 2021 at 1:42 PM EST, Pavel Stehule wrote:
> >
> > I found minor issues.
> >
> > Doc - missing tag
> >
> > and three whitespaces issues
> >
> > see attached patch
> Thanks, I need to remember to not skipp doc building for testing process
> 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 of 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 array.

My intention there was to highlight the difference between:

* SET obj['a']['b']['c'] = '"newvalue"'
* SET arr[0][0][3] = '"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. Take 2:

A <type>jsonb</type> value will accept assignments to nonexistent
subscript paths as long as object key subscripts can be traversed as
described above. The final subscript is not traversed and, if it
describes a missing object key, will be created. Nested arrays will
always be created and <literal>NULL</literal>-padded according to the
path until the value can be placed appropriately.

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