Re: [HACKERS] [PATCH] Generic type subscripting

st 7. 11. 2018 v 16:25 odesílatel Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
napsal:

> > On Fri, 12 Oct 2018 at 07:52, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
> >
> >> > postgres=# insert into test(v) values( '[]');
> >> > INSERT 0 1
> >> > postgres=# update test set v[1000] = 'a';
> >> > UPDATE 1
> >> > postgres=# update test set v[1000] = 'a';
> >> > UPDATE 1
> >> > postgres=# update test set v[1000] = 'a';
> >> > UPDATE 1
> >> > postgres=# select * from test;
> >> > ┌────┬─────────────────┐
> >> > │ id │ v │
> >> > ╞════╪═════════════════╡
> >> > │ │ ["a", "a", "a"] │
> >> > └────┴─────────────────┘
> >> > (1 row)
> >> >
> >> > It should to raise exception in this case. Current behave allows
> append simply, but can be source of errors. For this case we can introduce
> some special symbol - some like -0 :)
> >>
> >> Yeah, it may look strange, but there is a reason behind it. I tried to
> keep the
> >> behaviour of this feature consistent with jsonb_set function (and in
> fact
> >> they're sharing the same functionality). And for jsonb_set it's
> documented:
> >>
> >> If the item (of a path, in our case an index) is out of the range
> >> -array_length .. array_length -1, and create_missing is true, the
> new value
> >> is added at the beginning of the array if the item is negative, and
> at the
> >> end of the array if it is positive.
> >>
> >> So, the index 1000 is way above the end of the array v, and every new
> item has
> >> being appended at the end.
> >>
> >> Of course no one said that they should behave similarly, but I believe
> it's
> >> quite nice to have consistency here. Any other opinions?
> >
> >
> > Aha - although I understand to your motivation, I am think so it is bad
> design - and jsonb_set behave is not happy.
> >
> > I am think so it is wrong idea, because you lost some information -
> field position - I push value on index 10, but it will be stored on second
> position.
>
> The thing is that we don't store the field position in this sense anyway in
> jsonb. For arrays there are dimentions, boundaries and null bitmaps
> stored, but
> for jsonb it's just an array of elements. If we want to store this data, we
> either have to change the format, or fill in a jsonb with null values up
> to the
> required position (the first option is out of the scope of this patch, the
> second doesn't sound that good).
>

I don't agree. If we use a same syntax for some objects types, we should
to enforce some consistency.

I don't think so you should to introduce nulls for JSONs. In this case, the
most correct solution is raising a exception.

Regards

Pavel