1 How to improve SQL Performance with new Health Check Tool? Carlos Sierra Consulting Technical...

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How to improve SQL Performance with new Health Check Tool?Carlos SierraConsulting Technical Advisor

© 2012 Oracle Corporation – Proprietary and Confidential

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Topics

• What, Why, When and How?• Health Checks performed• What is the output?• Output sample• Demo• Q&A


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What is the SQL Health Check Tool?

• Small tool developed by the Server Technologies (ST) Center of Expertise (CoE)

• Stand-alone SQL script• SQL Health Check (HC) does not install schema

objects• Centric to one SQL statement per use• Requires to be executed connecting as SYS (or DBA)• Inputs one SQL_ID• Outputs HTML report with results of several Health

Checks and some additional information


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Why would I want to use the SQL HC?

• Easy way to verify environment around one SQL• Does not pollute the application nor the database• Executes in a few minutes• Minimum resources utilization• Documents “state” around one SQL• Safe to use on queries or DML statements• Acting upon output of relevant Health Checks

may improve execution plan thus performance• Output is small and easy to digest• Does not require advanced SQL Tuning expertise


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When should I use the SQL HC Tool?

• On a poorly performing SQL as per AWR or StatsPack reports

• On a SQL statement out of a Trace or TKPROF• When a quick sanity check of the environment around

one SQL is needed• When using SQLTXPLAIN (SQLT) MOS 215187.1

is not a short term option• When there had been recent changes on the system

and a SQL has regressed in terms of performance• When performance of a SQL is not consistent


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How do I use the SQL HC Tool?

• Download latest version of sqlhc.sql out of MOS 1366133.1

• Execute connected as SYS or a user with DBA privileges

• Parameters• Does your site have Oracle Diagnostics or Oracle Tuning

Pack licenses?• SQL_ID for the SQL statement to be analyzed

• Review and digest content of HTML output report• Act upon those “Observations” reported by HC

• Some may be more relevant than others in each case


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Topics



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Health Checks performed

• Levels• Global• Table• Index

• Purpose• Highlight SQL Tuning opportunities• Highlight concerns


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Global Health Checks

• Using “STATISTICS_LEVEL = ALL” on LINUX• CBO parameters with non-default values• optimizer_features_enable <> rdbms_version

• optimizer_dynamic_sampling between 1 and 3• db_file_multiblock_read_count is set• nls_sort is not binary

• DBMS_STATS automatic gathering• High version count• Using first rows• Fixed objects missing statistics


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• System statistics not gathered• mreadtim < sreadtim• sreadtim < 2• mreadtim < 3• sreadtim > 18• mreadtim > 522• SQL with policies as per v$vpd_policy• Materialized views with rewrite enabled• Table with bitmap index(es)• Index in plan no longer exists


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• SQL Plan Baseline contains non-reproducible Plans• Sizes of SQL Plan History and SQL Plan Baseline• SQL Profile combined VPD policies• Multiple CBO environments• INTERNAL_FUNCTION in a Filter Predicate• Plan Operations with Cost 0 and Cardinality 1• SREADTIM differs from actual db file sequential read• MREADTIM differs from actual db file scattered read


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Table Health Checks

• empty_blocks > blocks• Table DOP is set• Table has indexes with DOP set• Index degree <> Table degree• No statistics• No rows• Siebel small tables with statistics• Small sample size• Old statistics• Extended statistics


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Table Health Checks

• Columns with no statistics• Columns missing low/high values• Columns with old statistics• More nulls than rows• More distinct values than rows• Zero distinct values on columns with value• Incorrect NDV in long char column with histogram• Frequency histograms with less buckets than NDV• Frequency histogram with 1 bucket• Height balanced histogram with no popular values


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Table Health Checks

• ANALYZE command was used• Average row length is 100• Tables with stale statistics• SQL with policies as per dba_policies• SQL with policies as per dba_audit_policies• Table partitions with no statistics• Table partitions where num rows = 0• Table partitions with outdated statistics• Partitions with no column statistics• Partition columns with outdated statistics


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Table Health Checks

• Derived statistics in Tables or Partitions• Table has locked statistics


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Index Health Checks

• No statistics• More rows in index than its table• Clustering factor > rows in table• Statistics on zero while columns have value• Table/Index statistics out of sync• ANALYZE was used• No column statistics in single-column index• NDV on column > num_rows in single-column index• NDV is zero but column has values in single-column

index


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Index Health Checks

• NDV <> NDK in single-column index• Index partitions with no statistics• Index partitions where num rows = 0• Index partitions with outdated statistics• Derived statistics in Index or Partitions• Indexes with UNUSABLE segments• INVISIBLE Indexes• Index in a Plan with UNUSABLE segments• Index in a Plan is now INVISIBLE


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Topics



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What is the output?

• Identification• Observations (Health Checks results)• SQL Text• Tables Summary• Indexes Summary• Current SQL Statistics (GV$SQL)• Historical SQL Statistics (DBA_HIST_SQLSTAT)• Current Execution Plans (last execution)• Current Execution Plans (all executions)• Historical Execution Plans


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What is the output?

• Tables CBO Statistics• Columns CBO Statistics• Indexes CBO Statistics


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Observations section

• Global then Table then Index• All observations include a verbose explanation• Some observations have pointers to pertinent MOS

documents• Some are more relevant than others for your

particular SQL• Test any change before implementing in Production


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Tables Summary

• Content• Table level CBO statistics• Indexes and Columns counts• Columns with Histograms counts• Average sample sizes for Indexes and Columns

• Purpose• Quick quantitative view of Tables statistics health• Ease a compare between two systems believed to be similar


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Indexes Summary

• Content• Index level CBO statistics• Indication if Index is referenced in an Execution Plan• Columns counts• Columns with Histograms counts• Average sample sizes for Columns

• Purpose• Quick quantitative view of Indexes statistics health• Ease a compare between two systems believed to be similar


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Topics



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Output sample


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Output sample


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Output sample


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References

• MOS 1366133.1 SQL Tuning Health-Check Script • MOS 215187.1 SQLTXPLAIN (SQLT)


Date post:	21-Dec-2015
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