| From: | Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
|---|---|
| To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
| Cc: | 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: | 2020-09-17 13:56:27 |
| Message-ID: | 20200917135627.7koag6rnhshgjlai@localhost |
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| Lists: | pgsql-hackers |
> On Thu, Sep 17, 2020 at 02:47:54PM +0200, Pavel Stehule wrote:
> > I have my concerns about the performance side of this implementation as
> > well as how surprising this would be for users, but at the same time the
> > patch already does something similar and the code change should not be
> > that big, so why not - I can include this change into the next rebased
> > version. But it still can cause some confusion as it's not going to work
> > for negative indices, so
> >
> > update foo set a[-10] = 1;
> >
> > and
> >
> > select a[-10] from foo;
> >
> > can return different value from what was assigned. Otherwise, if we will
> > try to fix a[-10] assignment in the same way, it will prepend the array
> > and a[10] will not return the same value.
>
> What is semantic of negative index? It has clean semantic in C, but in
> PLpgSQL?
It's just a common pattern for jsonb when a negative index count from
the end of an array. I believe it was like that from the very earlier
implementations, although can't comment on that from the semantic point
of view.
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