Re: [HACKERS] SQL/JSON in PostgreSQL

On 07.01.2018 00:11, Pavel Stehule wrote:

2018-01-06 22:02 GMT+01:00 Oleg Bartunov <obartunov@gmail.com>:

On Sat, Jan 6, 2018 at 8:22 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> Hi
>
> I am checking the JSONPath related code
>
> Questions, notes:
>
> 1. jsonpath operators are not consistent with any other .. json, xml .. I am
> missing ?, @> operátors

I have slides about jsonpath
http://www.sai.msu.su/~megera/postgres/talks/sqljson-pgconf.eu-2017.pdf

> 2. documentation issue - there is "'{"a":[1,2,3,4,5]}'::json *? '$.a[*] ? (@
>> 2)'" - operator *? doesn't exists

There are should be @? operator

> 3. operator @~ looks like too aggressive shortcut - should be better
> commented
>
> What is not clean, if jsonpath should to create some new operators for json,
> jsonb types? It is special filter, defined by type, so from my perspective
> the special operators are not necessary.

It's impossible to distinguish jsonpath from text, so introducing new operators
are easier than everytime explicitly specify jsonpath datatype.

There are two possible solutions - special operator or explicit casting. In this case I am not sure if special operator for this case is good solution. Probably nobody will use it - because there SQL/JSON functions, but I don't think so this inconsistency is correct.

I have not strong opinion about it - it will be hidden feature for almost all users.

Operators are necessary for index support now.

Operators allows us to use a more concise syntax in simple cases, when we extract JSON item(s) without error handling:
js @* '$.key'
vs
JSON_QUERY(js, '$.key' RETURNING jsonb ERROR ON ERROR)


Also @* оperator gives us ability to extract a set of JSON items. JSON_QUERY can only wrap extracted item sequence into JSON array which we have to unwrap with our json[b]_array_elements() function. I also thought about returning setof-types in JSON_VALUE/JSON_QUERY:

JSON_QUERY(jsonb '[1,2,3]', '$[*]' RETURNING SETOF jsonb)

But it is not so easy to implement now, because we should introduce new node like TableFunc (or also we can try to use existing JSON_TABLE infrastructure).

Set-returning expressions are not allowed in every context, so for returning singleton items there should be additional operator.

--