tuning oracle sql - sfoug.org · the basics of efficient sql resorts on result after where and...

Tuning Oracle SQLTuning Oracle SQL

• The Basics of Efficient SQL• Common Sense Indexing• The Optimizer:

– Making SQL Efficient

• Finding Problem Queries• Oracle Enterprise Manager

– Wait Event Interface

• SELECTSELECT– FOR UPDATEFOR UPDATE

• Filtering– WHERE

• ORDER BY– often ignored– query complexity

• GROUP BY

The Basics of Efficient SQLThe Basics of Efficient SQLSELECT ** FROM division;SELECT division_id, name, city, state, country FROM division;SELECT division_id FROM division;

• SELECT– FOR UPDATE

• Filtering– WHEREWHERE

• ORDER BY– often ignored– query complexity

• GROUP BY

The Basics of Efficient SQLThe Basics of Efficient SQLAvoid unintentional full table scans

SELECT * FROM division WHERE country LIKE '%a%';

Match indexes Exact hits (equality)SELECT * FROM division WHERE division_id = 1;

Range scans / skip scans / full index scans

EXISTS (correlate) faster than IN

Biggest filters first

Full table scans can sometimes be faster

The Basics of Efficient SQLThe Basics of Efficient SQLResorts on result after WHERE and GROUP BY

Don’t repeat sorting (ORDER BY often ignored)

by SELECTSELECT division_id FROM division ORDER BY division_id;

by WHERESELECT * FROM division WHERE division_id < 10 ORDER BY division_id;

GROUP BYSELECT state, COUNT(state) FROM division GROUP BY state ORDER BY state;by DISTINCTSELECT DISTINCT(state) FROM division ORDER BY state;

by indexesSELECT division_id FROM division ORDER BY division_id;

• SELECT– FOR UPDATE

• Filtering– WHERE

• ORDER BYORDER BY– often ignored– query complexity

• GROUP BY

• SELECT– FOR UPDATE

• Filtering– WHERE

• ORDER BYORDER BY– often ignored– query complexity

• GROUP BY

The Basics of Efficient SQLThe Basics of Efficient SQLResorts on result after WHERE and GROUP BY

Don’t repeat sorting (ORDER BY often ignored)

by SELECTSELECT division_id FROM division ORDER BY division_id;

by WHERESELECT * FROM division WHERE division_id < 10 ORDER BY division_id;

GROUP BYSELECT state, COUNT(state) FROM division GROUP BY state ORDER BY state;by DISTINCTSELECT DISTINCT(state) FROM division ORDER BY state;

by indexesSELECT division_id FROM division ORDER BY division_id;

The Basics of Efficient SQLThe Basics of Efficient SQL

GROUP BYSELECT state, COUNT(state) FROM division GROUP BY state ORDER BY state;

HAVING (filters aggregate)SELECT state, COUNT(state) FROM division GROUP BY state HAVING COUNT(state) > 1;use WHERESELECT state, COUNT(state) FROM division WHERE state = 'NY' GROUP BY state;not HAVINGSELECT state, COUNT(state) FROM division GROUP BY state HAVING state = 'NY';

• SELECT– FOR UPDATE

• Filtering– WHERE

• ORDER BY– often ignored– query complexity

• GROUP BYGROUP BY– Use WHERE not HAVINGUse WHERE not HAVING

• SELECT– FOR UPDATE

• Filtering– WHERE

• ORDER BY– often ignored– query complexity

• GROUP BYGROUP BY– Use WHERE not HAVINGUse WHERE not HAVING

The Basics of Efficient SQLThe Basics of Efficient SQL

GROUP BYSELECT state, COUNT(state) FROM division GROUP BY state ORDER BY state;

HAVING (filters aggregate)SELECT state, COUNT(state) FROM division GROUP BY state HAVING COUNT(state) > 1;use WHERESELECT state, COUNT(state) FROM division WHERE state = 'NY' GROUP BY state;not HAVINGSELECT state, COUNT(state) FROM division GROUP BY state HAVING state = 'NY';

• FunctionsFunctions– conversions– miss indexes

– counteract with function based indexing– avoid using

– DECODE– CASE expressions– set operators (UNION)

• Use sequencesUse sequences• Use equality (=) or range scans (>)Use equality (=) or range scans (>)

– avoid negatives (!=, NOT)– avoid LIKE

The Basics of Efficient SQLThe Basics of Efficient SQL

The Basics of Efficient SQLThe Basics of Efficient SQL

• JoinsJoins– avoid Cartesian Products– avoid anti joins– avoid outer joins– perhaps replace

– multiple table complex joins– with subquery semi joins and inline views

• Be careful with viewsBe careful with views

Common Sense IndexingCommon Sense Indexing

• Don’t always need indexesDon’t always need indexes– table with few columns– static data– small tables– appended tables (SQL*Loader)

• How to indexHow to index– single column surrogate sequences– don’t override PK and FKs– avoid nullable columns

Common Sense IndexingCommon Sense Indexing

• Read write indexingRead write indexing– BTree

• Often read onlyOften read only– Bitmaps– IOTs– Clusters

Common Sense IndexingCommon Sense Indexing

• Read write indexingRead write indexing– BTree

– function based– can help a lot– get out of control

– everybody wants one– reverse key

– surrogate keys– High insertion rates

– not DW– Oracle RAC

Common Sense IndexingCommon Sense Indexing

• Often read onlyOften read only– Bitmaps

– can be much faster than BTrees – twice as fast in my book– at a previous client

– 1 year of DML activity– 100s of times slower– problem was nobody knew why– and nobody wanted to change anything

Common Sense IndexingCommon Sense Indexing

• Often read onlyOften read only– IOTs

– small number of columns– small tables– heard good things in Oracle RAC– even highly active DML environments

• Is intelligent– better with simple queries

• Is usually correct• Nothing is set in stone• Verify SQL code efficiency

– use EXPLAIN PLANEXPLAIN PLAN– SET AUTOTRACE ON EXPLAINSET AUTOTRACE ON EXPLAIN– $ORACLE_HOME/rdbms/admin/utlxplan.sql$ORACLE_HOME/rdbms/admin/utlxplan.sql

The OptimizerThe Optimizer

• Everything cost based– rule based is redundant

• Maintain statistics• Dynamic sampling

– OPTIMIZER_DYNAMIC_SAMPLING• Set TIMED_STATISTICS • Histograms

– maintain as for statistics– use for unevenly distributed indexes

The OptimizerThe Optimizer

• TablesTables– full Table scans

– small static tables– reading most of the rows

– over 10% for the Optimizer– reading deleted rows– parallel table scans

– sample table scans– SELECT * FROM generalledger SAMPLE(0.001);

– ROWID scans

The OptimizerThe Optimizer

• IndexesIndexes– index unique scan– index range scan

– reverse order index range scan

– index skip scan– index full scan

– fast full index scan

– others (very specific)

The OptimizerThe Optimizer

• JoinsJoins– nested loop join

– most efficient– row sets both small– one large and one small row set– one sorted

– hash join– both large with little difference– temporary hash table generated

The OptimizerThe Optimizer

• More on joinsMore on joins– sort merge join

– inefficient– both rows sets sorted then merge sorted

– other join types– semi joins– bitmap joins– star queries – Cartesian joins– outer joins (nested, hash or sort merge)

The OptimizerThe Optimizer

• Hints can change thingsHints can change things– influence the optimizer

• CURSOR_SHARING• FIRST or ALL_ROWS• DYNAMIC_SAMPLING

– change table scans• FULL

The OptimizerThe Optimizer

The OptimizerThe Optimizer• More on hintsMore on hints

– change index scans• INDEX_ASC, INDEX_DESC• INDEX_FFS• INDEX_JOIN (join indexes)• INDEX_SS_ASC or INDEX_SS_DESC• NO_INDEX, NO_INDEX_FFS or NO_INDEX_SS

– change joins• can change join type and influence with

parameters

– parallel SQL

Finding Problem QueriesFinding Problem Queries

• EXPLAIN PLANEXPLAIN PLAN– SET AUTOTRACE ON EXPLAIN– $ORACLE_HOME/rdbms/admin/utlxplan.sql$ORACLE_HOME/rdbms/admin/utlxplan.sql

• SQL Trace and TKPROF

Wait Event Wait Event InterfaceInterface

Finding Problem QueriesFinding Problem Queries• Performance views

• V$SQLAREA• executions• parsing• sorting• disk and buffer reads• fetching

• V$SQL• optimizer cost• CPU time• elapsed time

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Performance overview

Database health overview

Drilldown

More drilldown

TopSQL

EXPLAIN PLAN

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Wait Event InterfaceWait Event Interface

Use the help filesUse the HELP FILES in Use the HELP FILES in

Oracle Enterprise Oracle Enterprise ManagerManager