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.universality@gmail.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.klaver@aklaver.com