"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.
The syntax for json_exists belies this claim (assuming our docs are accurate there). Its "on error" options are true/false/unknown. Additionally, the predicate test operator is named "is unknown" not "is null".
The result of the predicate test, which is never produced as a value, only a concept, is indeed "unknown" - which then devolves to false when it is practically applied to determining whether to output the path item being tested. As it does also when used in a parth expression.
postgres=# select json_value('[null]','$[0] < 1'); json_value ------------ f
postgres=# select json_value('[null]','$[0] == null'); json_value ------------ t
Not sure how to peek inside the jsonpath system here though...
postgres=# select json_value('[null]','($[0] < 1) == null'); ERROR: syntax error at or near "==" of jsonpath input LINE 1: select json_value('[null]','($[0] < 1) == null');
I am curious if that produces true (the unknown is left as null) or false (the unknown becomes false immediately).
