Re: [HACKERS] [PATCH] Generic type subscripting

> 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.