postgres=# create schema a;
CREATE SCHEMA
postgres=# create schema b;
CREATE SCHEMA
postgres=# create schema c;
CREATE SCHEMA
postgres=# create function b.f () returns text as 'begin return a.f1(); end;' language plpgsql;
CREATE FUNCTION
postgres=# create function c.f () returns text as 'begin return a.f1(); end;' language plpgsql;
CREATE FUNCTION
postgres=# create function a.f1 () returns text as $$declare s text; begin get diagnostics s = PG_CONTEXT; return s; end;$$ language plpgsql;
CREATE FUNCTION
postgres=# select b.f();
                         f                          
----------------------------------------------------
 PL/pgSQL function a.f1() line 1 at GET DIAGNOSTICS+
 PL/pgSQL function b.f() line 1 at RETURN
(1 row)

postgres=# select c.f();
                         f                          
----------------------------------------------------
 PL/pgSQL function a.f1() line 1 at GET DIAGNOSTICS+
 PL/pgSQL function c.f() line 1 at RETURN
(1 row)

postgres=# set search_path = c,b,public;
SET
postgres=# select f();
                         f                          
----------------------------------------------------
 PL/pgSQL function a.f1() line 1 at GET DIAGNOSTICS+
 PL/pgSQL function c.f() line 1 at RETURN
(1 row)