Re: ​jsonb @@ jsonpath operator doc: ​Only the first item of the result is taken into account

On 4/2/23 06:21, Erik Wienhold wrote:
>> On 01/04/2023 08:02 CEST jian he <jian(dot)universality(at)gmail(dot)com> wrote:
>>
>> Hi,
>> https://www.postgresql.org/docs/current/functions-json.html
>>> jsonb @@ jsonpath → boolean
>>> Returns the result of a JSON path predicate check for the specified JSON
>>> value. Only the first item of the result is taken into account. If the
>>> result is not Boolean, then NULL is returned.
>>> '{"a":[1,2,3,4,5]}'::jsonb @@ '$.a[*] > 2' → t
>>
>> select jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*]');
>> return
>>
>>> jsonb_path_query
>>> ------------------
>>>  1
>>>  2
>>>  3
>>>  4
>>>  5
>>> (5 rows)
>>
>> I don't understand:"Only the first item of the result is taken into account.".
>>
>> Here,JSON path predicate check for the specified JSON valuereturn true, some
>> return false. (1 > 2 is false, 2 > 2 is false).
>
> The result is true if any array element matches the predicate because predicates
> are evaluated on sequences. The documentation for executePredicate in
> src/backend/utils/adt/jsonpath_exec.c explains it:
>
>> Predicates have existence semantics, because their operands are item
>> sequences. Pairs of items from the left and right operand's sequences are
>> checked. TRUE returned only if any pair satisfying the condition is found.
>> In strict mode, even if the desired pair has already been found, all pairs
>> still need to be examined to check the absence of errors. If any error
>> occurs, UNKNOWN (analogous to SQL NULL) is returned.
>
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/jsonpath_exec.c;h=b561f0e7e803f0e5a546ad118a47f625225b9708;hb=HEAD#l1461
>
> Difference between using a predicate as path expression vs filter expression:
>
> =# select jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*] > 2');
> jsonb_path_query
> ------------------
> true
> (1 row)
>
> =# select jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ > 2)');
> jsonb_path_query
> ------------------
> 3
> 4
> 5
> (3 rows)
>
> If you want the predicate result for each element, you must apply the predicate
> to the rows returned from jsonb_path_query:
>
> =# select elem, elem::float > 2 as pred from jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*]') elem;
> elem | pred
> ------+------
> 1 | f
> 2 | f
> 3 | t
> 4 | t
> 5 | t
> (5 rows)

That is a long way from:

jsonb @@ jsonpath → boolean

Returns the result of a JSON path predicate check for the specified JSON
value. Only the first item of the result is taken into account. If the
result is not Boolean, then NULL is returned.

>
> --
> Erik
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com