Date: Wed, 30 Dec 2020 14:45:12 +0100
From: Dmitry Dolgov <9erthalion6@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Pavel Stehule <pavel.stehule@gmail.com>,
	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 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.