Re: Patch: Improve Boolean Predicate JSON Path Docs

"David E. Wheeler" <david(at)justatheory(dot)com> writes:
> On Jan 20, 2024, at 12:34, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> It will take a predicate, but seems to always return true:
>>
>> regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] < 5' ;
>> ?column?
>> ----------
>> t
>> (1 row)
>>
>> regression=# select '{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] > 5' ;
>> ?column?
>> ----------
>> t
>> (1 row)

> Just for the sake of clarity, this return value is “correct,” because @? and other functions and operators that expect SQL standard statements evaluate the SET returned by the JSONPath statement, but predicate check expressions don’t return a set, but a always a single scalar value (true, false, or null). From the POV of the code expecting SQL standard JSONPath results, that’s a set of one. @? sees that the set is not empty so returns true.

I don't entirely buy this argument --- if that is the interpretation,
of what use are predicate check expressions? It seems to me that we
have to consider them as being a shorthand notation for filter
expressions, or else they simply do not make sense as jsonpath.

regards, tom lane