Re: Ambiguity in IS JSON description and logic

On Thu, Mar 20, 2025 at 7:22 AM Kirk Parker <khp(at)equatoria(dot)us> wrote:

> On Thu, Mar 20, 2025 at 7:08 AM Kirk Parker <khp(at)equatoria(dot)us> wrote:
>
>>
>> On Thu, Mar 20, 2025 at 2:46 AM PG Doc comments form <
>> noreply(at)postgresql(dot)org> wrote:
>>
>>> The following documentation comment has been logged on the website:
>>>
>>> Page: https://www.postgresql.org/docs/17/functions-json.html
>>> Description:
>>>
>>> On the manual page
>>> https://www.postgresql.org/docs/current/functions-json.html, in the
>>> Table
>>> 9.48. "SQL/JSON Testing Functions" there is a description of IS JSON. It
>>> includes the next sentence: "If WITH UNIQUE KEYS is specified, then any
>>> object in the expression is also tested to see if it has duplicate keys."
>>> And such text is ambiguous, because the term "object" has certain meaning
>>> regarding json format. In reality the option WITH UNIQUE KEYS allows to
>>> check for duplicated keys any array element not object. For objects, both
>>> WITH UNIQUE KEYS and WITHOUT UNIQUE KEYS return false, and both IS JSON
>>> ARRAY WITH UNIQUE KEY and IS JSON ARRAY WITHOUT UNIQUE KEY return true
>>> (it
>>> is at the same time with and without unique values, how it is possible?),
>>> i.e. it works the same as just IS JSON ARRAY. The example code that
>>> confirms
>>> my reasoning:
>>> SELECT
>>> js.vl AS "tested str",
>>>
>>>
>>> js.vl IS JSON OBJECT WITH UNIQUE KEYS AS ".. object w. UQ
>>> keys",
>>>
>>> js.vl IS JSON OBJECT WITHOUT UNIQUE KEYS AS ".. object w/o UQ
>>> keys",
>>> js.vl IS JSON ARRAY WITH UNIQUE KEYS AS ".. array w. UQ
>>> keys",
>>>
>>> js.vl IS JSON ARRAY WITHOUT UNIQUE KEYS AS ".. array w/o UQ keys",
>>> js.vl IS JSON ARRAY ".. array"
>>> FROM (VALUES ('{{"a": "a1"}, {"a": "a2"}}'), ('[{"a": "a1"}, {"a":
>>> "a2"}]'),
>>> ('["a", "a"]')) AS js(vl);
>>>
>>> I'm not sure what should be the right logic for this option, for me it
>>> looks
>>> now the same as simple IS JSON ARRAY without any UNIQUE KEY option, but
>>> if
>>> we use an option it should be either true for WITH UNIQUE KEYS or WITHOUT
>>> UNIQUE KEYS but not for both at the same time. But anyway the sentence I
>>> showed above should contain "array" instead of "object" because for
>>> objects
>>> it returns false independently of applied option. I tested it on
>>> "PostgreSQL 17.0 on x86_64-windows, compiled by msvc-19.41.34120,
>>> 64-bit".
>>>
>>
>> First, WITHOUT UNIQUE KEYS does not mean "confirm that there are
>> duplicate keys", it's just a way of stating the default explicitly. In
>> other words it means "w/o testing for duplicate keys". Thus IS JSON OBJECT
>> and IS JSON OBJECT WITHOUT UNIQUE KEYS will both always return identical
>> results on the same JSON expression.
>>
>> Secondly, the UNIQUE test is recursive; for objects maybe the meaning is
>> intuitive, but for JSON arrays -- which don't have any concept of keys;
>> JSON arrays are just ordered lists -- it means "does this array contain any
>> embedded objects with duplicate keys".
>>
>> See:
>>
>> SELECT js,
>> js IS JSON "json?",
>> js IS JSON OBJECT "object?",
>> js IS JSON OBJECT WITH UNIQUE KEYS "object w. UK?",
>> js IS JSON OBJECT WITHOUT UNIQUE KEYS "object w/o UK?",
>> js IS JSON ARRAY "array?",
>> js IS JSON ARRAY WITH UNIQUE KEYS "array w. UK?",
>> js IS JSON ARRAY WITHOUT UNIQUE KEYS "array w/o UK?"
>> FROM (VALUES
>> ('[{"a":1},{"b":2,"b":3}]'), -- expect t for array, array w/o UK
>> ('[{"a":1},{"b":2,"c":3}]'), -- expect t for ALL array tests
>> ('{"b":2,"b":3}'), -- expect t for object, object w/o UK
>> ('{"c":2,"d":3}'), -- expect t for ALL object tests
>> ('{"c":2,"d":{ "e": 0, "e": 1}}'), -- WITH UNIQUE is recursive for
>> nested objects
>> ('{"c":2,"d":{ "e": 0, "f": {"g":1,"g":2}}}'), -- no matter how deep
>> ('[{"a":1},{"b":2,"c":{"d":1, "d":2}}]') -- and also tests arrays
>> recursively for embedded objecs
>> ) foo(js);
>>
>>
>> A couple of side notes:
>>
>> 1. Your first data example is not JSON at all. It's helpful for this
>> kind of test to include a plain IS JSON column, since any of the IS JSON X
>> tests can fail for two reasons: (a) it's not JSON, or (b) it is JSON but
>> it's not an X.
>>
>> 2. Curiously, the JSON spec itself is completely silent on the meaning of
>> objects with duplicate keys. PostgreSQL is more helpful in this
>> regard--the docs explicitly state that the last value is the one that is
>> retained by JSONB and used in processing functions.
>>
>>
> To improve the documentation here, I would suggest simply adding the word
> "recursively" after "tested":
>
> If WITH UNIQUE KEYS is specified, then any object in the *expression* is
> also tested recursively to see if it has duplicate keys
>
>
I think the existing word "any" sufficiently implies "recursively". It
also doesn't really address the complaint here. I'm thinking something
more like:

(this is changed intentionally, see below)

expression IS [ NOT ] JSON [ { SCALAR | ARRAY | OBJECT } ] [ WITH UNIQUE ]
-> boolean

This predicate tests whether expression can be parsed as JSON. Two
additional properties can be tested at the same time: the type of the JSON
value, and whether it passes the unique object keys constraint. Enable the
first test by specifying one of SCALAR, ARRAY, or OBJECT. Enable the
second test by specifying WITH UNIQUE: This test is applied to all objects
contained within the JSON value. The return value is true only if
expression can be parsed as JSON and all enabled tests pass. The return
value is inverted if NOT is specified.

The test label "array w/o UK?" has to go. Coupling with the "additional
tests" idea introduced above, and the recommended syntax, we should do
something like:

SELECT js,
js IS JSON "parses ok, no tests",
js IS JSON OBJECT "object test only",
js IS JSON ARRAY "array test only",
js IS JSON WITH UNIQUE "unique test only",
js IS JSON ARRAY WITH UNIQUE "array and unique tests"

Then, to keep the technical reference thorough, re-add the full syntax at
the end.

This is the full syntax accepted for this predicate. Both VALUE and
WITHOUT, the default explicit keywords to disable the two additional tests,
as well as KEYS, are omitted above for clarity.
expression IS [ NOT ] JSON [ { VALUE | SCALAR | ARRAY | OBJECT } ] [ { WITH
| WITHOUT } UNIQUE [ KEYS ] ]

David J.