How to disable a SQL Plan Directive permanently

By Franck Pachot

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In 12c you will see a lot of SQL Plan Directives. Some are useful to get better execution plans, but some will trigger too much Adaptive Dynamic Sampling and that can become a big overhead, especially in Standard Edition. Then you don’t want to drop them – or they will reappear. You can disable them, but what will happen after the retention weeks? Let’s test it.

Disabled directive

A directive has been created which triggers too expensive dynamic sampling. You don’t want that and you have disabled it one year ago with:

SQL> dbms_spd.alter_sql_plan_directive(14130932452940503769,'ENABLED','NO');

and everything is good. You’re happy with that. Here is the directive:

SQL> select directive_id,state,last_used,auto_drop,enabled,extract(notes,'/spd_note/spd_text/text()') spd_text,extract(notes,'/spd_note/internal_state/text()') internal_state from dba_sql_plan_directives where directive_id in(select directive_id from dba_sql_plan_dir_objects where owner='DEMO' );

        DIRECTIVE_ID STATE      LAST_USED AUTO_DROP ENABLED SPD_TEXT                         INTERNAL_S
-------------------- ---------- --------- --------- ------- -------------------------------- ----------
14130932452940503769 SUPERSEDED 28-APR-14 YES       NO      {E(DEMO.DEMO_TABLE)[A, B, C, D]} HAS_STATS

The directive has not been used since April 2014 thanks to the ‘enabled’ set to NO.

If I run a query with a filter on those columns:

SQL> select count(*) Q1 from DEMO_TABLE where a+b=c+d;

                  Q1
--------------------
               10000

23:10:32 SQL> select * from table(dbms_xplan.display_cursor(format=>'basic +note'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select count(*) Q1 from DEMO_TABLE where a+b=c+d

Plan hash value: 1839825454

-----------------------------------------
| Id  | Operation          | Name       |
-----------------------------------------
|   0 | SELECT STATEMENT   |            |
|   1 |  SORT AGGREGATE    |            |
|   2 |   TABLE ACCESS FULL| DEMO_TABLE |
-----------------------------------------

there is no dynamic sampling that this is exactly what I want.

Retention weeks

My retention is the default: 53 weeks. Let’s see what happens after 53 weeks. I can call the ‘auto drop’ job with dbms_spd.drop_sql_plan_directive passing a null instead of a directive_id:

SQL> exec dbms_spd.drop_sql_plan_directive(null);

PL/SQL procedure successfully completed.

Run a few queries

Then let’s have a few queries on those table columns:

SQL> select count(*) Q2 from DEMO_TABLE where a+b=c+d;

                  Q2
--------------------
               10000

SQL> select count(*) Q3 from DEMO_TABLE where a+b=c+d;

                  Q3
--------------------
               10000

and check the execution plan:

SQL> select * from table(dbms_xplan.display_cursor(format=>'basic +note'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select count(*) Q3 from DEMO_TABLE where a+b=c+d

Plan hash value: 1839825454

-----------------------------------------
| Id  | Operation          | Name       |
-----------------------------------------
|   0 | SELECT STATEMENT   |            |
|   1 |  SORT AGGREGATE    |            |
|   2 |   TABLE ACCESS FULL| DEMO_TABLE |
-----------------------------------------

Note
-----
   - dynamic statistics used: dynamic sampling (level=2)
   - 1 Sql Plan Directive used for this statement

A directive has been used:

 SQL> select directive_id,state,last_used,auto_drop,enabled,extract(notes,'/spd_note/spd_text/text()') spd_text,extract(notes,'/spd_note/internal_state/text()') internal_state from dba_sql_plan_directives where directive_id in(select directive_id from dba_sql_plan_dir_objects where owner='DEMO' );

        DIRECTIVE_ID STATE      LAST_USED AUTO_DROP ENABLED SPD_TEXT                         INTERNAL_S
-------------------- ---------- --------- --------- ------- -------------------------------- ----------
14130932452940503769 SUPERSEDED 15-MAY-15 YES       YES     {E(DEMO.DEMO_TABLE)[A, B, C, D]} HAS_STATS

Oh! The directive is back and enabled !

Auto Drop

Here are the criteria for auto-drop. SPD are considered to be dropped when AUTO_DROP is YES and either:

• SPD is flagged as redundant
• One of the tables has been dropped (in recycle_bin means dropped)
• LAST_USAGE is from before the retention window
• State is NEW (LAST_USED is null) and CREATED is before retention window

Do you see? Nothing about the ENABLE YES/NO there…

Conclusion

If you want to disable a SPD and be sure that it will never reappear then you have to do both of following:

SQL> exec dbms_spd.alter_sql_plan_directive(14130932452940503769,'ENABLED','NO');
SQL> exec dbms_spd.alter_sql_plan_directive(14130932452940503769,'AUTO_DROP','NO');

then because the AUTO DROP is disabled, the directive will never be deleted automatically.