Re: [HACKERS] [PATCH] Generic type subscripting

From: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Oleksandr Shulgin <oleksandr(dot)shulgin(at)zalando(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, 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: 2020-11-30 11:34:54
Message-ID: 20201130113454.ohb6oho4fvwtpz3t@localhost
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> On Fri, Nov 27, 2020 at 12:13:48PM +0300, Alexander Korotkov wrote:
> Hi!
> I've started to review this patch.


> My first question is whether we're
> able to handle different subscript types differently. For instance,
> one day we could handle jsonpath subscripts for jsonb. And for sure,
> jsonpath subscripts are expected to be handled differently from text
> subscripts. I see we can distinguish types during in prepare and
> validate functions. But it seems there is no type information in
> fetch and assign functions. Should we add something like this to the
> SubscriptingRefState for future usage?
> Datum uppertypeoid[MAX_SUBSCRIPT_DEPTH];
> Datum lowertypeoid[MAX_SUBSCRIPT_DEPTH];

Yes, makes sense. My original idea was that it could be done within the
jsonpath support patch itself, but at the same time providing these
fields into SubscriptingRefState will help other potential extensions.

Having said that, maybe it would be even better to introduce a field
with an opaque structure for both SubscriptingRefState and
SubscriptingRef, where every implementation of custom subscripting can
store any necessary information? In case of jsonpath it could keep type
information acquired in prepare function, which would be then passed via
SubscriptingRefState down to the fetch/assign.

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