{ "date": "2015-06-12T08:01:13", "slug": "how-to-disable-a-sql-plan-directive-permanently", "link": "https://www.dbi-services.com/blog/how-to-disable-a-sql-plan-directive-permanently/", "title": { "rendered": "How to disable a SQL Plan Directive permanently" }, "content": { "rendered": "

By Franck Pachot

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\nIn 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.

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Disabled directive

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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:

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SQL> dbms_spd.alter_sql_plan_directive(14130932452940503769,'ENABLED','NO');\n
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and everything is good. You’re happy with that. Here is the directive:

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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' );\n\n        DIRECTIVE_ID STATE      LAST_USED AUTO_DROP ENABLED SPD_TEXT                         INTERNAL_S\n-------------------- ---------- --------- --------- ------- -------------------------------- ----------\n14130932452940503769 SUPERSEDED 28-APR-14 YES       NO      {E(DEMO.DEMO_TABLE)[A, B, C, D]} HAS_STATS\n
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The directive has not been used since April 2014 thanks to the ‘enabled’ set to NO.

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

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SQL> select count(*) Q1 from DEMO_TABLE where a+b=c+d;\n\n                  Q1\n--------------------\n               10000\n\n23:10:32 SQL> select * from table(dbms_xplan.display_cursor(format=>'basic +note'));\n\nPLAN_TABLE_OUTPUT\n------------------------------------------------------------------------------------\nEXPLAINED SQL STATEMENT:\n------------------------\nselect count(*) Q1 from DEMO_TABLE where a+b=c+d\n\nPlan hash value: 1839825454\n\n-----------------------------------------\n| Id  | Operation          | Name       |\n-----------------------------------------\n|   0 | SELECT STATEMENT   |            |\n|   1 |  SORT AGGREGATE    |            |\n|   2 |   TABLE ACCESS FULL| DEMO_TABLE |\n-----------------------------------------\n\n
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there is no dynamic sampling that this is exactly what I want.

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Retention weeks

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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:

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SQL> exec dbms_spd.drop_sql_plan_directive(null);\n\nPL/SQL procedure successfully completed.\n
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Run a few queries

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

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SQL> select count(*) Q2 from DEMO_TABLE where a+b=c+d;\n\n                  Q2\n--------------------\n               10000\n\nSQL> select count(*) Q3 from DEMO_TABLE where a+b=c+d;\n\n                  Q3\n--------------------\n               10000\n\n
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and check the execution plan:

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SQL> select * from table(dbms_xplan.display_cursor(format=>'basic +note'));\n\nPLAN_TABLE_OUTPUT\n--------------------------------------------------------------------------\nEXPLAINED SQL STATEMENT:\n------------------------\nselect count(*) Q3 from DEMO_TABLE where a+b=c+d\n\nPlan hash value: 1839825454\n\n-----------------------------------------\n| Id  | Operation          | Name       |\n-----------------------------------------\n|   0 | SELECT STATEMENT   |            |\n|   1 |  SORT AGGREGATE    |            |\n|   2 |   TABLE ACCESS FULL| DEMO_TABLE |\n-----------------------------------------\n\nNote\n-----\n   - dynamic statistics used: dynamic sampling (level=2)\n   - 1 Sql Plan Directive used for this statement\n\n
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A directive has been used:

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 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' );\n\n        DIRECTIVE_ID STATE      LAST_USED AUTO_DROP ENABLED SPD_TEXT                         INTERNAL_S\n-------------------- ---------- --------- --------- ------- -------------------------------- ----------\n14130932452940503769 SUPERSEDED 15-MAY-15 YES       YES     {E(DEMO.DEMO_TABLE)[A, B, C, D]} HAS_STATS\n
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Oh! The directive is back and enabled !

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Auto Drop

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

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

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Conclusion

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

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SQL> exec dbms_spd.alter_sql_plan_directive(14130932452940503769,'ENABLED','NO');\nSQL> exec dbms_spd.alter_sql_plan_directive(14130932452940503769,'AUTO_DROP','NO');\n
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then because the AUTO DROP is disabled, the directive will never be deleted automatically.

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