Re: [HACKERS] [PATCH] Generic type subscripting

čt 17. 9. 2020 v 15:56 odesílatel Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
napsal:

> > 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.
>

ok, then I think we can design some workable behaviour

My first rule - there should not be any implicit action that shifts
positions in the array. It can be explicit, but not implicit. It is true
for positive indexes, and it should be true for negative indexes too.

then I think so some like this can work

if (idx < 0)
{
if (abs(idx) > length of array)
exception("index is of of range");
array[length of array - idx] := value;
}
else
{
/* known behave for positive index */
}

Regards

Pavel