Using SQL Plan Management (SPM) to balance Plan Flexibility and Plan Stability

Carlos Sierra

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Carlos Sierra• SQLTXPLAIN + SQL Health-Check SQLHC +• Consultant/Developer/DBA/Design/+• Oracle Performance + SQL Tuning• Oracle Database Health-Check• Tools + Scripts• Speaker

QuestionEverything

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Topics• Motivation• SQL Plan Management (SPM)– Mechanics– Demos

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Plan Flexibility• Cost-based Optimizer (CBO)– Introduced in Oracle 7– Computes an Optimal Plan based on • Heuristics• Query Predicates• Schema Object Statistics• Several Others

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Flexibility: Good or Evil?• CBO Mission– Compute an Optimal Plan for a given SQL and its Data

• Plans have many and complex Dependencies• CBO Performance– Successful most of the times• Produces a sub-optimal Plan sometimes

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DBA Recurrent Nightmare• A business critical process experiences an

occasional slow down– Same SQL usually runs quickly– Developers claim they haven’t changed anything– Users claim they do the same every time– Business keeps reporting this: to YOU!

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Why Plans Perform Inconsistently? (1)• Concurrency with some other Process• State of Buffer Cache• RAC Node Affinity• CPU Starvation• Sudden Changes on Data Volume• Inconsistent Performance through Database Link

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Why Plans Perform Inconsistently? (2)• Downgraded Parallel Execution• Crossing Small-Table Threshold• Resource Manager Directive• Plan Flips• Many Others…

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Why a Plan Flips? (1)• Bind Variable Peeking with Histograms• Outdated Schema Object (SO) Statistics• Missing SO Statistics• Fresh SO Statistics• New Empty Partitions

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Why a Plan Flips? (2)• Incomplete set of CBO Hints– Either on SQL Text or SQL Profile

• Dropped, Invalid, Invisible or new Indexes• Index or Table rebuild• Two or more Plans with similar Cost

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Why a Plan Flips? (3)• Database Parameters Changes• CBO System Statistics Changes• Asynchronous SO Statistics Gathering• Many Others…

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How to Mitigate Plan Flipping?• Solid CBO Statistics• Default CBO Parameters• Avoid Global Changes

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Plan Flexibility Allies• CBO Parameters• CBO Statistics• Dynamic Sampling• Cardinality Feedback• Cursor Sharing• Adaptive Cursor Sharing (ACS)

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Plan Stability Tools• CBO Hints• Stored Outlines• SQL Profiles• SQL Plan Management (SPM)

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SQL Plan Management• 11g+• Multiple Persistent Optimal Execution Plans per

SQL• Only “accepted” and “enabled” Plans are Executed• New Plans are acknowledged but not Executed• Goal: Plan Stability with controlled Flexibility

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SQL Plan Baseline• A set of Plans available to the CBO for a given SQL– Identified by SQL Handle and Signature • Hash function on SQL Text• dbms_sqltune.sqltext_to_signature

– View dba_sql_plan_baselines• enabled = YES• accepted = YES• reproduced = YES

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Plan History• Content of dba_sql_plan_baselines• Includes SQL Plan Baseline • Includes Pending Plans– accepted = NO and last_verified is NULL

• Includes Rejected Plans– accepted = NO and last_verified is not NULL

• Includes Disabled Plans

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Methods to Create a Plan Baseline

1. Capture2. Load

a. Cursor Cache (CUR)b. SQL Tuning Set (STS)c. Stored Outline

3. Migration

1. Capture2. Load

a. CURb. STSc. AWRd. SPAe. TRC

3. Migration

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Capturing a SQL Plan Baseline (1)• Set optimizer_use_sql_plan_baselines to TRUE

(default)• Set optimizer_capture_sql_plan_baselines to TRUE

(default is FALSE)– Set this parameter at SESSION level

• Execute SQL 2 times• Set optimizer_capture_sql_plan_baselines to FALSE

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Capturing a SQL Plan Baseline (2)

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Loading SQL Plan Baseline from Cache• dbms_spm.load_plans_from_cursor_cache– Inputs• sql_id• plan_hash_value (opt)

– Outputs• Number of Plans loaded

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DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE ( sql_id IN VARCHAR2, plan_hash_value IN NUMBER := NULL, sql_text IN CLOB, fixed IN VARCHAR2 := 'NO', enabled IN VARCHAR2 := 'YES') RETURN PLS_INTEGER;

DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE ( sql_id IN VARCHAR2, plan_hash_value IN NUMBER := NULL, sql_handle IN VARCHAR2, fixed IN VARCHAR2 := 'NO', enabled IN VARCHAR2 := 'YES') RETURN PLS_INTEGER;

DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE ( sql_id IN VARCHAR2, plan_hash_value IN NUMBER := NULL, fixed IN VARCHAR2 := 'NO', enabled IN VARCHAR2 := 'YES') RETURN PLS_INTEGER;

DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE ( attribute_name IN VARCHAR2, attribute_value IN VARCHAR2, fixed IN VARCHAR2 := 'NO', enabled IN VARCHAR2 := 'YES') RETURN PLS_INTEGER;

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One of possible attribute names:• 'SQL_TEXT' (like)• 'PARSING_SCHEMA_NAME'

(equality)• 'MODULE' (equality)• 'ACTION' (equality)

SQL text to use in identifying the SQL plan baseline into which the plans are loaded. If the SQL plan baseline does not exist, it is created. The use of text is crucial when the user tunes a SQL statement by adding hints to its text and then wants to load the resulting plan(s) into the SQL plan baseline of the original SQL statement.SQL handle to use in identifying the SQL plan baseline into which the plans are loaded. The sql_handle must denote an existing SQL plan baseline. The use of handle is crucial when the user tunes a SQL statement by adding hints to its text and then wants to load the resulting plan(s) into the SQL plan baseline of the original SQL statement.

Plan identifier. Default NULL means capture all plans present in the cursor cache for the SQL statement identified by SQL_ID.

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SQL Plan Baseline Loading Options• Load a Plan from a Hinted version of a SQL into SQL

Baseline of Original SQL– Use SQL Text if there is no pre-existing SQL Baseline– Use SQL Handle if SQL Baseline already exists

• Load one or more Cursor Plans (memory) for a SQL• Load Plans for Subset of Cursors based on Schema,

SQL Text “like” or Application (Module or Action)

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Demo 6• Load Plans for Query out of Demo 5– demo5.sql (1st time)– load_plans.sql

• Execute Demo 5 again and review Plan History– demo5.sql (2nd time)– list_plans.sql

• Execute demo5.sql 2x (3rd & 4th) and verify results

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Demo 6 Results (after 3rd demo5.sql) CHILD EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE SHAR SENS AWRE----------- ----------- ----------- --------------- ---- ---- ---- 0 3 9090 3600618656 Y Y N

CHILD BUCKET_ID COUNT----------- ----------- ----------- 0 0 0 0 1 1 0 2 2Note----- - SQL plan baseline SQL_PLAN_652hmt7yxthdwc0e59472 used for this statement

CREATED PLAN_NAME ENA ACC REP FIX------------------------------ ------------------------------ --- --- --- ---21-JUN-14 02.00.10.000000 PM SQL_PLAN_652hmt7yxthdw80508062 YES YES YES NO21-JUN-14 02.00.10.000000 PM SQL_PLAN_652hmt7yxthdwc0e59472 YES YES YES NO21-JUN-14 02.00.10.000000 PM SQL_PLAN_652hmt7yxthdwe00bee24 YES YES YES NO21-JUN-14 02.00.19.000000 PM SQL_PLAN_652hmt7yxthdwc82a6b0c YES NO YES NO

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Demo 6 Results (after 4th demo5.sql) CHILD EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE SHAR SENS AWRE----------- ----------- ----------- --------------- ---- ---- ----

0 2 723482 847574763 N Y N 1 1 2404 3600618656 Y Y Y 2 1 1977 189372815 Y Y Y 3 1 568 847574763 Y Y Y

Note----- - SQL plan baseline SQL_PLAN_652hmt7yxthdw80508062 used for this statement

CREATED PLAN_NAME ENA ACC REP FIX------------------------------ ------------------------------ --- --- --- ---28-DEC-13 02.03.43.000000 PM SQL_PLAN_652hmt7yxthdw501f1424 YES YES YES NO28-DEC-13 02.03.43.000000 PM SQL_PLAN_652hmt7yxthdwb6c39290 YES YES YES NO28-DEC-13 02.03.43.000000 PM SQL_PLAN_652hmt7yxthdwd624c0cd YES YES YES NO28-DEC-13 02.03.50.000000 PM SQL_PLAN_652hmt7yxthdwc82a6b0c YES NO YES NO

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Demo 7• Drop SQL Plan Baseline– drop_plans.sql

• Create SQL Patch with /*+ BIND_AWARE */ Hint– sqlpch.sql for 8u0n7w1jug5dg connected as SYS

• Load Plans for Query out of Demo 5– demo5.sql– load_plans.sql

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Demo 7 Results CHILD EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE SHAR SENS AWRE----------- ----------- ----------- --------------- ---- ---- ----

0 1 2191 847574763 N Y Y 1 1 3030 2048551027 Y Y Y 2 1 2404 3600618656 Y Y Y 3 1 1977 189372815 Y Y Y 4 1 568 847574763 Y Y Y

CREATED PLAN_NAME ENA ACC REP FIX------------------------------ ------------------------------ --- --- --- ---21-JUN-14 02.09.14.000000 PM SQL_PLAN_652hmt7yxthdw80508062 YES YES YES NO21-JUN-14 02.09.14.000000 PM SQL_PLAN_652hmt7yxthdwc0e59472 YES YES YES NO21-JUN-14 02.09.14.000000 PM SQL_PLAN_652hmt7yxthdwc82a6b0c YES YES YES NO21-JUN-14 02.09.14.000000 PM SQL_PLAN_652hmt7yxthdwe00bee24 YES YES YES NO

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Demo 8• Execute queries 1-5 and observe how number of

Executions increases per Child Cursor– flush.sql– demo8.sql– demo8.sql– demo8.sql

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Demo 8 Results CHILD EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE SHAR SENS AWRE----------- ----------- ----------- --------------- ---- ---- ----

0 1 3030 2048551027 Y Y Y 1 1 2404 3600618656 Y Y Y 2 1 1977 189372815 Y Y Y 3 1 568 847574763 N Y Y 4 1 724 847574763 Y Y Y

Note----- - SQL patch "sqlpch_8u0n7w1jug5dg" used for this statement - SQL plan baseline SQL_PLAN_652hmt7yxthdwb6c39290 used for this statement

CHILD EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE SHAR SENS AWRE----------- ----------- ----------- --------------- ---- ---- ----

0 2 6060 2048551027 Y Y Y 1 2 4808 3600618656 Y Y Y 2 2 3954 189372815 Y Y Y 3 1 568 847574763 N Y Y 4 3 2016 847574763 Y Y Y

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Evolving a Plan (1)• Evolving a Plan means “accepting” it– Promoting it from Plan History into SQL Plan Baseline

• dbms_spm.evolve_sql_plan_baseline– sql_handle (opt)– plan_name (opt)– verify (default YES)– commit (default YES)

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DBMS_SPM.EVOLVE_SQL_PLAN_BASELINE ( sql_handle IN VARCHAR2 := NULL, plan_name IN VARCHAR2 := NULL, time_limit IN INTEGER := DBMS_SPM.AUTO_LIMIT, verify IN VARCHAR2 := 'YES', commit IN VARCHAR2 := 'YES') RETURN CLOB;

DBMS_SPM.EVOLVE_SQL_PLAN_BASELINE ( plan_list IN DBMS_SPM.NAME_LIST, time_limit IN INTEGER := DBMS_SPM.AUTO_LIMIT, verify IN VARCHAR2 := 'YES', commit IN VARCHAR2 := 'YES') RETURN CLOB;

Evolving a Plan (2)

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Demo 9• Drop SQL Plan Baseline– drop_plans.sql

• Capture SQL Plan Baseline with 1 accepted Plan– demo9.sql– list_plans.sql

• Evolve all Plans on Plan History– evolve.sql

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FIXED Flag• When set to YES– Only FIXED Plans are considered for Plan Selection– No more Plans are Captured into Plan History

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Alter SQL Plan BaselinesDBMS_SPM.ALTER_SQL_PLAN_BASELINE ( sql_handle IN VARCHAR2 := NULL, plan_name IN VARCHAR2 := NULL, attribute_name IN VARCHAR2, attribute_value IN VARCHAR2) RETURN PLS_INTEGER;

Attribute Names & Values:enabled 'YES' or 'NO'fixed 'YES' or 'NO'autopurge 'YES' or 'NO'plan_name String of up to 30-charactersdescription String of up to 500-characters

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Plan Selection (1)• At hard parse when SQL Plan Baseline (SPB) exists

1. CBO computes an “Optimal” Plan (OP)• Adaptive Cursor Sharing is used if applicable

2. If OP exists in SPB then execute this OP3. If OP does not exist in SPB then store it in Plan History

a. If there are FIXED Plans in SPBi. Re-cost all FIXED Plans using Binds and execute cheapest

b. If there are no FIXED Plans in SPMi. Re-cost all Plans using Binds and execute cheapest

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Plan Selection (2)

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Demo 10• FIX one Plan from SQL Plan Baseline– alter_plans.sql

• Verify only one Plan was Executed– flush.sql– demo8.sql– demo8.sql

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SPM Summary• SPM provides much desired Plan Stability– Only “approved” Plans are allowed to Execute

• ACS can generate a healthy stock of Optimal Plans– You can restrict them to work under SPM

• Combining ACS and SPM you can obtain– Plan Flexibility and Plan Stability

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ACS Suggested Strategy• Use sys.dbms_sqldiag_internal.i_create_patch to

SQL Patch with BIND_AWARE the SQL on Baselines • Use free script sqlpch.sql

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SPM Suggested Strategies

1. Conservative– One SQL at a time

2. Moderate– One Application at a time (Module or Action)– One Schema at a time

3. Aggressive (a.k.a. Cowboy method)– All Application Schemas at once

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Oracle Pack Licenses• DBMS_SPM– No Oracle Pack License

• DBMS_SQLTUNE– Oracle Tuning Pack

• AWR– Oracle Diagnostics Pack

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References (1)• Random Slowdown by Jonathan Lewis– http://jonathanlewis.wordpress.com/2013/12/23/plan-

changes/• SQL Plan Management SPM Aware Optimizer– https://blogs.oracle.com/optimizer/entry/

sql_plan_management_part_2_of_4_spm_aware_optimizer

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References (2)• Oracle® Database PL/SQL Packages and Types

Reference– 11g Release 2 (11.2)– Part Number E25788-04

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Contact Information• carlos.sierra@enkitec.com• carlos-sierra.net• @csierra_usa