hi.
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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:
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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.
