hi. ------------- 9.16.2.1.1. Boolean Predicate Check Expressions As an extension to the SQL standard, a PostgreSQL path expression can be a Boolean predicate, whereas the SQL standard allows predicates only within filters. 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. For example, we could write this SQL-standard filter expression:
------------- slight inconsistency, "SQL-standard" versus "SQL standard" "path expression can be a Boolean predicate", why capital "Boolean"? "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.
