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(at)gmail(dot)com
> <mailto:obartunov(at)gmail(dot)com>>:
>
> On Sat, Jan 6, 2018 at 8:22 AM, Pavel Stehule
> <pavel(dot)stehule(at)gmail(dot)com <mailto:pavel(dot)stehule(at)gmail(dot)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
> <http://www.sai.msu.su/%7Emegera/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.

--
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company