SQL>  select * from (
  2         select fPrint('error ' || t.doc_id) as bug from DOCS t
  3          where t.doc_id = 50128384 and t.doc_type_id < 0
  4  ) p where p.bug is not null;
BUG
--------------------------------------------------------------------------------
error 50128384

Select 
   fPrint('error ' || t.doc_id) as bug 
 From DOCS t
 Where 
         t.doc_id = 50128384 and t.doc_type_id < 0
  And fPrint('from where ' || t.doc_id /* || t.doc_type_id */) is not null

SQL> select /*+ RULE */ count(*) from (
  2           select fPrint('error ' || t.doc_id) as bug from DOCS t
  3            where t.doc_id = 50128384 and t.doc_type_id < 0
  4    ) p where p.bug is not null
  5  /
  COUNT(*)
----------
         0
error 50128384

Description                           Object Name   ID  Instance  Object Type    Operation          Optimizer     Options         Parent ID  Position  Search columns
SELECT STATEMENT, GOAL = HINT: RULE                  0                           SELECT STATEMENT   HINT: RULE
 SORT AGGREGATE                                      1                           SORT                             AGGREGATE          0         1
  TABLE ACCESS BY INDEX ROWID           DOCS         2      2     TABLE          TABLE ACCESS       ANALYZED      BY INDEX ROWID     1         1
   INDEX UNIQUE SCAN                  PK__DOCS       3            INDEX (UNIQUE) INDEX              ANALYZED      UNIQUE SCAN        2         1          1

 Select /*+ RULE */
SQL> set serveroutput on
SQL> 
SQL>  Select /*+ RULE */
  2     besapi.fPrint('error ' || t.doc_id) as bug
  3   From DOCS t
  4   Where
  5           t.doc_id = 50128384 and t.doc_type_id < 0
  6    And besapi.fPrint('or ' || t.doc_id) is not null
  7  /
BUG
--------------------------------------------------------------------------------
 Эта строка ручной работы. Тут ничего не было, т.к. ноль строк. Следующая строка это печать из функции. 
or 50128384

SQL> dbms_xplan

select /*+ no_merge */ count(*) from (
         select /*+ no_merge */ fPrint('error ' || t.doc_id) as bug from DOCS t
          where t.doc_id = 50128384 and t.doc_type_id < 0
  ) p where p.bug is not null

with v_no_rows as (select /*+ no_merge */ fPrint('error ' || t.doc_id) as bug from DOCS t
                   where t.doc_id = 50128384 and t.doc_type_id < 0)

select /*+ no_merge */ count(*) from v_no_rows p where p.bug is not null