Meet the CBO in Version 11g
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The document discusses advanced features and optimizations in Oracle Database, focusing on Cost-Based Optimization (CBO) in versions 10g and 11g, including the use of bind peeking and adaptive cursors. It addresses performance improvements and specific strategies for handling SQL execution plans and statistics gathering, along with recommendations for best practices in coding and database configuration. The document emphasizes the importance of managing SQL plan baselines and the evolving nature of optimizer performance across different Oracle versions.
Meet the CBO in Version 11g
- 1. www.sagecomputing.com.au penny@sagecomputing.com.au Meet the Cost Based Optimiser in 11g Penny Cookson SAGE Computing Services SAGE Computing Services Customised Oracle Training Workshops and Consulting
- 2. SAGE Computing Services Customised Oracle Training Workshops and Consulting Penny Cookson Managing Director and Principal Consultant Working with Oracle products since 1987 Oracle Magazine Educator of the Year 2004 www.sagecomputing.com.au penny@sagecomputing.com.au
- 3. A Characteristic Problem I havenât changed anything Its really slow this morning I did the same thing yesterday and it was fine Actually its OK now No its not Thank you so much youâve fixed it (I havenât done anything)
- 4. Oracle Version < 9 SELECT COUNT(l.quantity) FROM bookings_skew l WHERE resource_code = :v1; âBRLGâ <Version 9 database No bind peeking Shared Pool SELECT COUNT(l.quantity) FROM bookings_skew_large l WHERE resource_code = :v1; âPC1â FULL SCAN How many rows do I expect?
- 5. Oracle Version < 9 SELECT COUNT(l.quantity) FROM bookings_skew l WHERE resource_code = :v1; âBRLGâ <Version 9 database No bind peeking Shared Pool SELECT COUNT(l.quantity) FROM bookings_skew_large l WHERE resource_code = :v1; âPC1â FULL SCAN How many rows do I expect?
- 6. What is Bind Peeking? SELECT COUNT(l.quantity) FROM bookings_skew l WHERE resource_code = :v1; âPC1â >=Version 9 database Bind peeking Shared Pool SELECT COUNT(l.quantity) FROM bookings_skew l WHERE resource_code = :v1; âBRLGâ INDEXED ACCESS How many rows do I expect?
- 7. Histogram â Minority First *************************************** SINGLE TABLE ACCESS PATH Column (#3): RESOURCE_CODE(VARCHAR2) AvgLen: 5.00 NDV: 9 Nulls: 0 Density: 9.0892e-008 Histogram: Freq #Bkts: 9 UncompBkts: 5966 EndPtVals: 9 Table: BOOKINGS_SKEW Alias: L Card: Original: 5464800 Rounded: 43968 Computed: 43967.55 Non Adjusted: 43967.55 Access Path: TableScan Cost: 7693.35 Resp: 7693.35 Degree: 0 Cost_io: 7426.00 Cost_cpu: 1555877511 Resp_io: 7426.00 Resp_cpu: 1555877511 Access Path: index (AllEqRange) Index: BK_RESSKEW resc_io: 2399.00 resc_cpu: 37397785 ix_sel: 0.0080456 ix_sel_with_filters: 0.0080456 Cost: 2405.43 Resp: 2405.43 Degree: 1 Best:: AccessPath: IndexRange Index: BK_RESSKEW Cost: 2405.43 Degree: 1 Resp: 2405.43 Card: 43967.55 Bytes: 0 *************************************** RESO COUNT(*) ---- ---------- VCR1 495711 CONF 495720 LNCH 743576 BRSM 743583 PC1 47858 FLPC 495720 BRLG 1739277 TAP1 247864 VCR2 495715
- 8. Histogram â Minority First
- 9. What is the CBO good at in 10g? Data Condition Literal/Bind Var Histogram Even Distribution Equality Literal N/A Even Distribution Equality Bind N/A Skewed Equality Literal NO Skewed Equality Literal YES Skewed Equality Bind NO Skewed Equality Bind YES Even Distribution Range Bind N/A
- 10. Bind Peeking + Adaptive Cursors in 11g Statements with bind variables (+histograms) are bind sensitive The first time you execute a statement with different selectivity it uses the original plan The second time it changes the plan and become bind aware New values will use a plan for the appropriate selectivity range
- 13. PLSQL has soft parse avoidance Implicit cursor Explicit cursor Native dynamic SQL Ref cursor Session_cached_cursors = 0 No default adaptive cursor functionality Default adaptive cursor functionality
- 14. Adaptive Cursors Functionality Bind variable with Equality and Histogram Not for range conditions Bind variable with Equality and Histogram Range conditions Does not support LIKE /*+ BIND_AWARE */ Adaptive Cursors 11.1.0.6 Adaptive Cursors 11.1.0.7 and 11.2
- 15. So where are we now? Our own code â works properly first time with hint in SQL and PL/SQL Packages â will still get it wrong once and wonât use adaptive cursors for PL/SQL at all unless we set session cached cursors =0
- 24. xxxxxxxxxxxxxxxxxxxxx Maybe you should buy an Exadata box
- 25. Bind Peeking + Adaptive Cursors Summary Your code Packages Minimise statement invalidations Consider running the statement with minority and majority value on start up Use the BIND_AWARE hint (SQL and PL/SQL) for skewed data Use ref cursors (if hints are not allowed) ?manually hack an outline/profile PL/SQL â set session_cached_cursors = 0 (temporarily)
- 26. Adaptive Cursors What we Really Need Once a statement has been bind aware it knows next time its parsed ?SQL *Profile to indicate bind aware Adaptive Cursors Persistence
- 27. Try to get rid of your hints _optimizer_ignore_hints Create additional statistics Set _optimizer_ignore_hints to TRUE Test the app If it runs OK remove your hints
- 28. 58 New Hints SELECT name, inverse, sql_feature, version FROM v$sql_hint WHERE version like '11%' ORDER BY version desc, name New Hints
- 29. New Hints IGNORE_ROW_ON_DUPKEY_INDEX This has nothing to do with optimisation I just like it
- 30. New Hints MONITOR NO_MONITOR select /*+ MONITOR */ count(comments) from bookings ; select DBMS_SQLTUNE.REPORT_SQL_MONITOR( session_id=>sys_context('userenv','sid'), report_level=>'ALL') as report from dual;
- 31. Much More Query Transformation SELECT e.event_no, e.start_date, sum(cost) totcost FROM events_large e, bookings_large b WHERE e.event_no = b.event_no GROUP BY e.event_no, e.start_date alter session set tracefile_identifier = Penny alter session set events '10053 trace name context forever' explain plan for SELECT e.event_no, e.start_date, sum(cost) totcost FROM events_large e, bookings_large b WHERE e.event_no = b.event_no GROUP BY e.event_no, e.start_date; alter session set events '10053 trace name context off'
- 32. Much More Query Transformation SELECT e.event_no, e.start_date, sum(cost) totcost FROM events_large e, bookings_large b WHERE e.event_no = b.event_no GROUP BY e.event_no, e.start_date
- 33. 10G â JOIN before the GROUP BY
- 35. 11G â GROUP BY before the JOIN
- 41. 10G â NOT IN (PROMISE_NO NULLABLE)
- 42. 10G â KILLED AFTER 1 hr
- 43. 11G â Rewrite as Null Aware Anti join
- 45. 10G â Full Outer Join â default behaviour
- 47. From version 10.2.0.3 SELECT /*+ NATIVE_FULL_OUTER_JOIN */ count(e.comments) nume, count (b.comments) numb FROM events_large e FULL OUTER JOIN bookings_large b ON (e.event_no = b.event_no) 10G â Full Outer Join â hint
- 48. 11G Native Full Outer Join
- 50. Check the differences in plans Upgrading
- 54. Oracle 10g â Run a whole load of random statements â all of which require parsing
- 55. Oracle 11g â Run a whole load of random statements â all of which require parsing So Tuning the Shared Pool becomes more important, but overall the 11g CBO is pretty smart
- 56. Results Cache SGA populate invalidate Most recently used SQL/PL/SQL SQL Query Results Cache Results Cache PL/SQL Function Results Cache Most recently used result sets Buffer Cache Library Cache Most recently data
- 57. Query Results Cache RESULT_CACHE_MODE MANUAL FORCE /*+RESULT_CACHE */ /*+ NO_RESULT_CACHE */ SELECT /*+ RESULT_CACHE */ count(b.comments) FROM train1.events_large e, train1.bookings_large b WHERE e.org_id = :v1 AND e.event_no = b.event_no AND e.comments = :v2;
- 58. PL/SQL Function Results Cache CREATE OR REPLACE FUNCTION quantity_booked (p_resource_code in resources.code%TYPE,p_event_date in date) RETURN NUMBER RESULT_CACHE IS v_total_booked number := 0; BEGIN SELECT sum(b.quantity) INTO v_total_booked FROM bookings b, events e WHERE e.event_no = b.event_no AND p_event_date between e.start_date and e.end_date AND b.resource_code = p_resource_code; RETURN (v_total_booked); END;
- 59. Monitoring the Results Cache SELECT * FROM v$result_cache_memory SELECT * FROM v$result_cache_objects SELECT * FROM v$result_cache_statistics SELECT * FROM v$result_cache_dependency RESULT_CACHE_MAX_SIZE RESULT_CACHE_MAX_RESULT Explain Plan
- 61. Setting Statistics Gathering Defaults GET_PARAM SET_PARAM Procedure GET_PREFS Function SET_TABLE_PREFS (set for individual tables) SET_DATABASE_PREFS (sets for all tables, can include/exclude SYS tables) SET_GLOBAL_PREFS â for new objects Obsolete: Use:
- 62. Gathering Statistics Early CBO: âMake sure you gather statistics regularlyâ Later CBO: âDonât gather statistics unless data patterns changeâ BUT If you have new majority values you need to recreate the histogram
- 63. Gathering and Publishing Set preferences to PUBLISH = âFALSEâ Gather Statistics user_tab_pending_stats user_ind_pending_stats user_col_pending_stats Run test case Alter session set optimizer_pending_statistics = TRUE Worse Better dbms_stats. delete_pending_stats dbms_stats. publish_pending_stats user_tab_statistics user_ind_statistics user_tab_col_statistics
- 64. Gathering and Publishing SELECT obj#, TO_CHAR(savtime,'dd/mm/yyyy') save_time, rowcnt, blkcnt, avgrln, samplesize, analyzetime FROM wri$_optstat_tab_history ORDER BY savtime desc
- 65. CBO - Instability CBO ?access path decision System parameters Session parameters Optimiser StatisticsIndexes SQL Profile Software version System statistics DBA playing around
- 66. CBO - Instability â Leave it all alone â SQL Plan Management â Store plan baseline â Plans not used till accepted â Manually accept or â Allow Oracle to evolve plans Solution 1 Solution 2
- 67. SQL Plan Management Manual capture DBMS_SPM.LOAD_PLANS_FROM_SQLSET DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE Auto capture of repeatable statements OPTIMIZER_CAPTURE_SQL_PLAN_BASELINE = TRUE Baseline = (Stored and Accepted plans) SQL Management Base New Plan identified during execution Stored not accepted SQL Tuning Advisor identifies new plan â SQL*Profile accepted Auto accept of new plan (if it performs better) DBMS_SPM.EVOLVE_SQL_PLAN_BASELINE Manual load/accept of new plan DBMS_SPM.LOAD_PLANS_FROM_SQLSET DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE
- 69. SAGE Computing Services Customised Oracle Training Workshops and Consulting Questions? www.sagecomputing.com.au penny@sagecomputing.com.au