AND_EQUAL hint ignored

Question

Hi,
in my 9.2.0.8 I've got query like:
SELECT /*+ AND_EQUAL(l I_LOK_OPJ_FK I_LOK_DGR_FK) USE_NL(L) USE_NL(OPJ)*/ MAX(lok_audyt_dt)
                    FROM  LIMITY_OKRESY l,  OSOBA_PROGRAM_LOJAL opj,  OSOBY oss
                    WHERE lok_opj_id = opj_id
                    AND opj_osb_id = osb_id
                    AND osb_audyt_st = '1'
                    AND opj_audyt_st = '1'
                    AND lok_audyt_st = '1'
                    and osb_audyt_st = opj_audyt_st
                    and osb_audyt_st = lok_audyt_st
                    and lok_dgr_id = 26
                    AND oss.osb_pesel =  '33010511133'
| Id  | Operation                      |  Name                    | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT               |                          |     1 |    57 | 75898 |
|   1 |  SORT AGGREGATE                |                          |     1 |    57 |       |
|   2 |   NESTED LOOPS                 |                          |   929 | 52953 | 75898 |
|   3 |    NESTED LOOPS                |                          |   900 | 34200 |  1198 |
|   4 |     TABLE ACCESS BY INDEX ROWID|  OSOBY                |    92 |  2024 |    94 |
|   5 |      INDEX RANGE SCAN          | I_OSB_PESEL            |    92 |       |     3 |
|   6 |     TABLE ACCESS BY INDEX ROWID|  OSOBA_PROGRAM_LOJAL  |    10 |   160 |    12 |
|   7 |      INDEX RANGE SCAN          | I_OPJ_SYNCHRO2         |    10 |       |     3 |
|   8 |    TABLE ACCESS BY INDEX ROWID |  LIMITY_OKRESY        |     1 |    19 | 75898 |
|   9 |     AND-EQUAL                  |                          |       |       |       |
|  10 |      INDEX RANGE SCAN          | I_LOK_OPJ_FK           |  1363K|       |    80 |
|  11 |      INDEX RANGE SCAN          | I_LOK_DGR_FK           |  1363K|       |    80 |
Note: cpu costing is off, PLAN_TABLE' is old version
    explain plan for
    SELECT /*+ AND_EQUAL(l I_LOK_OPJ_FK I_LOK_DGR_FK) */ MAX(lok_audyt_dt)
                        FROM  LIMITY_OKRESY l,  OSOBA_PROGRAM_LOJAL opj,  OSOBY oss
                        WHERE lok_opj_id = opj_id
                        AND opj_osb_id = osb_id
                        AND osb_audyt_st = '1'
                        AND opj_audyt_st = '1'
                        AND lok_audyt_st = '1'
                        and osb_audyt_st = opj_audyt_st
                       and osb_audyt_st = lok_audyt_st
                       and lok_dgr_id = 26
                       AND oss.osb_pesel =  '33010511133'
Explained.
SQL> select * from table(dbms_xplan.display());
| Id  | Operation                       |  Name                    | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT                |                          |     1 |    57 |  1844 |
|   1 |  SORT AGGREGATE                 |                          |     1 |    57 |       |
|   2 |   HASH JOIN                     |                          |   929 | 52953 |  1844 |
|   3 |    TABLE ACCESS BY INDEX ROWID  |  OSOBA_PROGRAM_LOJAL  |    10 |   160 |    12 |
|   4 |     NESTED LOOPS                |                          |   900 | 34200 |  1198 |
|   5 |      TABLE ACCESS BY INDEX ROWID|  OSOBY                |    92 |  2024 |    94 |
|   6 |       INDEX RANGE SCAN          | I_OSB_PESEL            |    92 |       |     3 |
|   7 |      INDEX RANGE SCAN           | I_OPJ_SYNCHRO2         |    10 |       |     3 |
|   8 |    TABLE ACCESS BY INDEX ROWID  |  LIMITY_OKRESY        |  1363K|    24M|   630 |
|   9 |     INDEX RANGE SCAN            | I_LOK_DGR_FK           |  1363K|       |    80 |
Note: cpu costing is off, PLAN_TABLE' is old versionThe first query which respects AND_EQUAL hint runns with 243 cr , the second one which somehow ignores the hint need about 40k cr to complete .
Why CBO is ignoring my and_equal hint I need that for performance .
Please advice.
Regards.
Greg

Answer 1

user10388717 wrote:
Hi,
in my 9.2.0.8 I've got query like:
(snip)
Note: cpu costing is off, PLAN_TABLE' is old version
The first query which respects AND_EQUAL hint runns with 243 cr , the second one which somehow ignores the hint need about 40k cr to complete .
Why CBO is ignoring my and_equal hint I need that for performance .
Please advice.
Regards.
GregGreg,
I do not see anything that would explicitly prevent the AND_EQUAL hint from working, other than an automatic transformation of the SQL statement that only happened when the two nested loop hints were provided. Hints are directives - the optimizer must obey hints unless:
* The hint is invalid due to the wrong alias used in the hint
* The hint is malformed
* The hint is incompatible with another hint
* The query was transformed by the optimizer into a form that is incompatible with the hint before the optimizer applied the hint
* The hint, if followed, would cause the wrong results to be returned
See this article for more information:
http://hoopercharles.wordpress.com/2010/07/19/demonstration-of-oracle-ignoring-an-index-hint/
I would assume that bullet point #4 applies in this case. Examining a 10053 trace, as suggested by riedelme, might help. I believe that the 9i version of a 10053 trace will show how the query is being transformed as it is optimized, just like 10g and above.
Note that the AND_EQUAL hint is deprecated as of Oracle 10g. Jonathan Lewis' "Cost-Based Oracle Fundamentals" book contains a little bit of information about that hint and the AND EQUAL execution path:
http://books.google.com/books?id=TGSd3pkMx5IC&pg=PA457
Charles Hooper
Co-author of "Expert Oracle Practices: Oracle Database Administration from the Oak Table"
http://hoopercharles.wordpress.com/
IT Manager/Oracle DBA
K&M Machine-Fabricating, Inc.