Have a table column with JSON string and a (defined) function (get_jstr()) that returns a single sting.

SELECT get_jstr();
<..returns some JSON string...>

How do i feed this output to jsonb_path_query() function?
Tried a few variants

SELECT jsonb_path_query( (SELECT get_jstr()), '$[0]');
SELECT jsonb_path_query( v, '$[0]') FROM (SELECT get_jstr()) v;

but none of them work. Any suggestions?