Re: [HACKERS] [PATCH] Generic type subscripting

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, 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: 2020-12-09 00:34:03
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Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2020-12-07 17:25:41 -0500, Tom Lane wrote:
>> I can see that that should work for the two existing implementations
>> of EEO_CASE, but I wasn't sure if you wanted to wire in an assumption
>> that it'll always work.

> I don't think it's likely to be a problem, and if it ends up being one,
> we can still deduplicate the ops at that point...

Seems reasonable.

Here's a v38 that addresses the semantic loose ends I was worried about.
I decided that it's worth allowing subscripting functions to dictate
whether they should be considered strict or not, at least for the fetch
side (store is still assumed nonstrict always) and whether they should be
considered leakproof or not. That requires only a minimal amount of extra
code. While the planner does have to do extra catalog lookups to check
strictness and leakproofness, those are not common things to need to
check, so I don't think we're paying anything in performance for the
flexibility. I left out the option of "strict store" because that *would*
have required extra code (to generate a nullness test on the replacement
value) and the potential use-case seems too narrow to justify that.
I also left out any option to control volatility or parallel safety,
again on the grounds of lack of use-case; plus, planner checks for those
properties would have been in significantly hotter code paths.

I'm waiting on your earlier patch to rewrite the llvmjit_expr.c code,
but otherwise I think this is ready to go.

regards, tom lane

Attachment Content-Type Size
v38-generic-subscripting-core-feature.patch text/x-diff 140.0 KB

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