Re: BUG #16022: to_json on arrays with unusual lower bound is not intuitive

Hi!

On 25.09.19 16:34, Tom Lane wrote:
> PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
>> I would expect the array indices of a JSON array to match with the
>> subscripts of a postgres array when converting them back and forth.
>> However, Postgres lets the JSON array begin at the `lower_bound` of the
>> array, not at subscript 1 (= index 0).
>> For example, `to_json('[-2:5]={-2,-1,0,1,2,3,4,5}'::int[])` results in
>> `[-2,-1,0,1,2,3,4,5]` where I would have expected `[1,2,3,4,5]` (or an
>> error, since JSON arrays must not have negative indices).
>
> I can see no reason whatever for either of those definitions to be
> better than the established one. If you want some other conversion
> rule, write your own function that behaves the way you want.

OK, I agree it's kinda unreasonable to change this.

However, I would like to see this behaviour documented at
https://www.postgresql.org/docs/current/functions-json.html, so FUP'd to
pgsql-docs.

I propose to add the following description to Table 9.45:

| The resulting JSON array starts at the lower bound of the PostgreSQL
array, regardless whether that is 1 or not.

Maybe also add `to_json('[3:8]={3,4,5,6,7,8}'::int[])` becoming
`[3,4,5,6,7,8]` to the examples.

While we're at it, I would also like to see the documentation of
`array_lower` and `array_upper` to be improved in
https://www.postgresql.org/docs/current/functions-array.html. I was a
bit surprised that when passing an empty array, they did return `NULL`
instead of the "default" value `1`. Can you add that as an example to
the two table rows, or just add a simple "(NULL if empty)" parenthetical
to their descriptions?
Alternatively, document it after the table:

| In `array_lower` and `array_upper`, `NULL` is returned if the array
dimension does not exist or is empty.

kind regards,
Andreas Bergmaier