Re: minor doc issue in 9.16.2.1.1. Boolean Predicate Check Expressions

On Thu, Oct 17, 2024 at 7:59 AM Bruce Momjian <bruce@momjian.us> wrote:
>
>
> Where are we on this?  I still see this behavior.
>
> ---------------------------------------------------------------------------

> > but I found following two examples returning different results,
> > i think they should return the same value.
> > select json_value('1', '$ == "1"' returning jsonb error on error);
> > select json_query('1', '$ == "1"' returning jsonb error on error);

This part has been resolved.
see section Note section in
https://www.postgresql.org/docs/current/functions-json.html#SQLJSON-QUERY-FUNCTIONS


>
> On Fri, Jun 21, 2024 at 04:53:55PM +0800, jian he wrote:
> > On Fri, Jun 21, 2024 at 11:11 AM David G. Johnston
> > <david.g.johnston@gmail.com> wrote:
> > >
> > > On Thu, Jun 20, 2024 at 7:30 PM jian he <jian.universality@gmail.com> wrote:
> > >>
> > >> "predicate check expressions return the single three-valued result of
> > >>
> > >> the predicate: true, false, or unknown."
> > >> "unknown" is wrong, because `select 'unknown'::jsonb;` will fail.
> > >> here "unknown" should be "null"? see jsonb_path_query doc entry also.
> > >>

doc (https://www.postgresql.org/docs/devel/functions-json.html#FUNCTIONS-SQLJSON-CHECK-EXPRESSIONS)
<<QUOTE>>
While SQL-standard path expressions return the relevant element(s) of
the queried JSON value, predicate check expressions return the single
three-valued result of the predicate: true, false, or unknown.
<<END OF QUOTE>>

https://www.postgresql.org/docs/current/datatype-boolean.html
says
"The boolean type can have several states: “true”, “false”, and a
third state, “unknown”, which is represented by the SQL null value."

but here
select jsonb_path_query('1', '$ == "a"');
return JSON null value, not SQL null value.

however.
select jsonb_path_match('1', '$ == "a"');
return SQL null value.


maybe we can change to
"predicate check expressions return the single three-valued result of
the predicate: true, false, or null"

Then in the <note> section mention that
when Predicate check expressions cannot be applied, it returns JSON
null for function jsonb_path_query,
return SQL NULL for function jsonb_path_match or @@ operator.