Subquery removal using window functions

Question

Hi,
DB is 9.2.0.8 EE .
select * from FROM bt_osoby osoby , bt_osoby_firmy osf1
WHERE osoby.osb_audyt_st = '1'
  AND osf1.osf_audyt_st = '1'
  AND osoby.osb_id = osf1.osf_osb_id
  AND osoby.osb_nzm_id != 4140963  
  AND osoby.osb_id = ( SELECT MAX (osoby2.osb_id)
                         FROM bt_osoby osoby2
                         WHERE osoby2.osb_audyt_st = '1'
                         AND osoby2.osb_nzm_id = osoby.osb_nzm_id) I'm trying to rewrite above with subquery removal using window functions trick.
select * from bt_osoby osoby , (
  SELECT first_value(osoby2.osb_id) over (partition by osoby2.osb_nzm_id order by osoby2.osb_id desc nulls last) max_osb_id ,osoby2.osb_nzm_id o2_osb_nzm_id
                         FROM bt_osoby osoby2
                         WHERE osoby2.osb_audyt_st = '1') v
                         ,bt_osoby_firmy osf1
                         where
                         osoby.osb_audyt_st = '1'
  AND osf1.osf_audyt_st = '1'
  AND osoby.osb_id = osf1.osf_osb_id
  AND osoby.osb_nzm_id != 4140963
  AND   v.max_osb_id = osoby.osb_id
  AND osoby.osb_nzm_id = v.o2_osb_nzm_idbut rewritten query plan involves huge FTS on BT_OSOBY (much worst then NL over sub query before), how to
benefit from 'subquery removal using window functions' then?
Regards.
GG

Answer 1

Hi,
As you discovered, a query with a sub-query can be more efficient that one without a sub-query.
I would do this using the analytic RANK function (or, depending on your needes, maybe ROW_NUMBER).
WITH     bt_osoby_with_rnum     AS
     select  bt_osoby.*
     ,     RANK () OVER ( PARTITION BY  osb_nzm_id
                           ORDER BY          osb_id     DESC
                    ) AS rnum
     FROM      bt_osoby
     WHERE     osb_audyt_st     = '1'
     AND     osb_nzm_id      != 4140963  
SELECT     *     -- or list all columns except rnum
FROM     bt_osoby_wtih_rnum             osoby
,      bt_osoby_firmy                 osf1
WHERE      osf1.osf_audyt_st  = '1'
AND      osoby.osb_id = osf1.osf_osb_id
;Of course, without versions of your tables, I can't test this.
Note that this does use a sub-query.