Date: Wed, 30 Dec 2020 21:01:37 +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 30, 2020 at 07:48:57PM +0100, Pavel Stehule wrote:
> st 30. 12. 2020 v 14:46 odesílatel Dmitry Dolgov <9erthalion6@gmail.com>
> napsal:
>
> > > On Wed, Dec 30, 2020 at 02:45:12PM +0100, Dmitry Dolgov wrote:
> > > > On Sat, Dec 26, 2020 at 01:24:04PM -0500, Tom Lane wrote:
> > > >
> > > > In a case like jsonpath['...'], the initially UNKNOWN-type literal
> > could
> > > > in theory be coerced to any of these types, so you'd have to resolve
> > that
> > > > case manually.  The overloaded-function code has an internal preference
> > > > that makes it choose TEXT if it has a choice of TEXT or some other
> > target
> > > > type for an UNKNOWN input (cf parse_func.c starting about line 1150),
> > but
> > > > if you ask can_coerce_type() it's going to say TRUE for all three
> > cases.
> > > >
> > > > Roughly speaking, then, I think what you want to do is
> > > >
> > > > 1. If input type is UNKNOWNOID, choose result type TEXT.
> > > >
> > > > 2. Otherwise, apply can_coerce_type() to see if the input type can be
> > > > coerced to int4, text, or jsonpath.  If it succeeds for none or more
> > > > than one of these, throw error.  Otherwise choose the single successful
> > > > type.
> > > >
> > > > 3. Apply coerce_type() to coerce to the chosen result type.
> > > >
> > > > 4. At runtime, examine exprType() of the input to figure out what to
> > do.
> > >
> > > Thanks, that was super useful. Following this suggestion I've made
> > > necessary adjustments for the patch. There is no jsonpath support, but
> > > this could be easily added on top.
> >
> > And the forgotten patch itself.
> >
>
> make check fails

Yeah, apparently I forgot to enable asserts back after the last
benchmarking discussion, and missed some of those. Will fix.

> 2. The index position was ignored.
>
> postgres=# update foo set a['a'][10] = '20';
> UPDATE 1
> postgres=# select * from foo;
> ┌─────────────┐
> │      a      │
> ╞═════════════╡
> │ {"a": [20]} │
> └─────────────┘
> (1 row)

I just realized I haven't included "filling the gaps" part, that's why
it works as before. Can add this too.

> 1. quietly ignored update
>
> postgres=# update foo set a['a'][10] = '20';
> UPDATE 1
> postgres=# select * from foo;
> ┌────┐
> │ a  │
> ╞════╡
> │ {} │
> └────┘
> (1 row)

This belongs to the original jsonb_set implementation. Although if we
started to change it anyway with "filling the gaps", maybe it's fine to
add one more flag to tune its behaviour in this case as well. I can
check how complicated that could be.