Re: [PATCH] WIP: replace method for jsonpath

> On 18 Sep 2024, at 11:23 AM, Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> On 17.09.24 21:16, David E. Wheeler wrote:
>> On Sep 17, 2024, at 15:03, Florents Tselai <florents(dot)tselai(at)gmail(dot)com> wrote:
>>> Fallback scenario: make this an extension, but in a first pass I didn’t find any convenient hooks.
>>> One has to create a whole new scanner, grammar etc.
>> Yeah, it got me thinking about the RFC-9535 JSONPath "Function Extension" feature[1], which allows users to add functions. Would be cool to have a way to register jsonpath functions somehow, but I would imagine it’d need quite a bit of specification similar to RFC-9535. Wouldn’t surprise me to see something like that appear in a future version of the spec, with an interface something like CREATE OPERATOR.
>
> Why can't we "just" call any suitable pg_proc-registered function from JSON path? The proposed patch routes the example '$.replace("hello","bye")' internally to the internal implementation of the SQL function replace(..., 'hello', 'bye'). Why can't we do this automatically for any function call in a JSON path expression?
>

Well, we can.
A couple of weeks ago, I discovered transform_jsonb_string_values, which is already available in jsonfuncs.h
and that gave me the idea for this extension https://github.com/Florents-Tselai/jsonb_apply

It does exactly what you’re saying: searches for a suitable pg_proc in the catalog, and directly applies it.

select jsonb_apply('{
"id": 1,
"name": "John",
"messages": [
"hello"
]
}', 'replace', 'hello', 'bye’);

select jsonb_filter_apply('{
"id": 1,
"name": "John",
"messages": [
"hello"
]
}', '{messages}', 'md5’);

But, I don't know… jsonb_apply? That seemed “too fancy”/LISPy for standard Postgres.

Now that you mention it, though, there’s an alternative of tweaking the grammar and calling the suitable text proc.