Re: [HACKERS] [PATCH] Generic type subscripting

> 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).