Dale Roberts04/19/23 1

Department of Computer and Information Science,School of Science, IUPUI

Dale Roberts, Lecturer

Computer Science, IUPUI

E-mail: droberts@cs.iupui.edu

SQL Tuning IntroductionSQL Tuning Introduction

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Oracle Data Dictionary

Data Dictionary:stores all information that is used to manage the objects in the database.

Nomenclature:USER_% objects you ownALL_% objects you own, have been

granted access to, or publicDBA_% all objects in database.

Road Map Tables:DICTIONARY (DICT) - describes the Oracle data dictionary tables.

select table_name, comments from DICTwhere table_name like ‘USER_%’;

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• Tables about Database Objects:– USER_CATALOG (CAT) - tables, views, synonyms, sequences– USER_OBJECTS (OBJ) - clusters, links, functions, indexes, packages,

procedures, sequences, synonyms, tables, triggers, views– USER_TABLES (TABS)– USER_TAB_COLUMNS (COLS)– USER_VIEWS– USER_SYNONYMS (SYN)– USER_SEQUENCES (SEQ)

• Constraints:– USER_CONSTRAINTS– USER_CONS_COLUMNS– EXCEPTIONS

• Comments:– USER_TAB_COMMENTS– USER_COL_COMMENTS

• Indexes:– USER_INDEXES (IND)– USER_IND_COLUMNS

• Clusters:– USER_CLUSTERS (CLU)– USER_CLU_COLUMNS

Oracle Data Dictionary

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• Links, Snapshots:– USER_DB_LINKS– USER_SNAPSHOTS– USER_SNAPSHOT_LOGS

• Procedures:– USER_TRIGGERS– USER_SOURCE– USER_ERRORS– USER_OBJECT_SIZE– USER_ARGUMENTS

• Space Allocation:– USER_TABLESPACES– USER_SEGMENTS– USER_EXTENTS– USER_FREE_SPACE

• User Tables: – USER_USERS– USER_RESOURCE_LIMITS– USER_TAB_PRIVS– USER_SYS_PRIVS

• Roles:– USER_ROLE_PRIVS– ROLE_TAB_PRIVS - DBA privilege required– ROLE_SYS_PRIVS - DBA privilege required– ROLE_ROLE_PRIVS - DBA privilege required

Oracle Data Dictionary

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• Auditing: (only if auditing turned on by DBA)– USER_AUDIT_OBJECT– USER_AUDIT_SESSION– USER_AUDIT_STATEMENT– USER_OBJ_AUDIT_OPTS

• Dynamic Tables: (DBA ONLY)– V$ACCESS - locks currently held on objects– V$FILESTAT - file read/write statistics– V$LOCK - non-DDL locks and resources– V$PROCESS - currently active processes– V$SESSION - currently active sessions– V$SGA - size of each component in the SGA (System Global Area)– V$SYSSTAT - current statistics on entire database– V$TRANSACTION- info about transactions.

• Chained Rows:– Create table CHAINED_ROWS

from $ORA_HOME/rdbms/admin/utlchain.sql– Analyze a table like this:

analyze LEDGER list chained rows into CHAINED_ROWS;• Plan Table:

– Create table PLAN_TABLE from $ORA_HOME/rdbms/admin/utlxplan.sql

– Analyze a query like this:explain plan set Statement_id = ‘A’ for select ...;

Oracle Data Dictionary

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• Understand the Applications Tasks – What data is to be captured? – How should the data be processed?– How should the data be reported?

Good Design Has a Human Touch

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• Ways to Increase DB Performance:– Relax normalization rules by using meaningful keys, pp. 609-612– Put redundant data into 3NF tables, pp. 612-614– More memory, p. 614– Combining smaller tables,

“Kitchen Junk Drawer” approach, p. 615– Computation Table, p. 616– Snapshots, Chapter 23

Performance and Design

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• Why tune SQL statements:– Improve response time of interactive programs.– Improve batch throughput.– To ensure scalability of applications (load v. perf.)– Reduce system load for other uses besides DB.– Avoid hardware upgrades.

• Scalability:– Well-tuned applications deliver good performance as number of

users or data volume increases.– Applications which have a linear degradation pattern degrade

predictably, usually they are problems responsive to hardware upgrades.

– Exponential degradation patterns are more serious. They tend to be problems that creep up over time, starting benign and unrecognized, later becoming huge problems.

– Bottlenecks are performance problems which are abrupt, like “hitting a brick wall”. Usually no warning, and no hardware solution.

SQL Tuning

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• Objections to SQL tuning:– “The Oracle optimizer will tune my statements for me

automatically.”– “I’m not a SQL programmer, I’m a ... VB/PowerBuilder/C++/Java

programmer.”– “I’ll write the SQL, someone else (DBA) can tune it for me later.”– “I’ll tune it later”– “We can’t afford to tune it”

• When to tune:– Early is least costly & better. – Changing SQL/table designs in the design phase means that no

applications need to be re-written.– Tuning SQL performance when SQL is first written usually

means lower testing costs.– In production systems, testing SQL can sometimes be difficult...

change control, production system availability, extra time to deal with larger data volumes.

– Tune SQL early as possible to be most effective and economical.

SQL Tuning

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Prepare for Tuning :gather statistics

Finished

SQLOptimized?

Gather Data: Explain plan SQL_TRACE and

tk_prof

Tune the SQL: Reword SQL Use Hints Add or change indexes or clusters Alter table structure(s)

SQL Tuning Process

Yes

No

SQL Tuning

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SQL Tuning

• Retrieving Data:– Full Table Scan - gets data from row 1 to high water mark– Row ID - gets data by physical location. Quickest way to get a

row.– Index Lookup - matches up key value with Row ID – Hash Key Lookup - computes a Row ID with a mathematical

formula applied to key value.

• Joining Data:– Sort Merge Join

• sorts each table’s key columns• merges data together• does not use indexes

– Nested Loops Join• full table scan used on smaller table• key values of 1st table joined with the larger table’s index

– Hash Join• hash key is built for larger table, constructs index on the fly• smaller table is then fully scanned• data from smaller table is joined with the hash key index.

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SQL Tuning

• Oracle Optimizers:– Rule Based Optimizer

• older optimizer, used since first versions of Oracle.• set of rules which rank access paths.• always picks an index over doing full table scan.

– Cost Based Optimizer• new in Oracle 7.• takes advantage of database statistics to pick optimal access path.• To collect table statistics:

ANALYZE TABLE tablename CALCULATE STATISTICS;

• Optimizer Goals:– RULE, picks only rule based optimizer.– CHOOSE, picks cost based optimizer if any table in query has been

analyzed.– ALL_ROWS, picks the cost based optimizer and finds an execution plan

which is best for the entire query. Good for batch reporting.– FIRST_ROWS, pick the cost based optimizer and finds an execution

plan which is best for the first row. Good for interactive applications.

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SQL Tuning

• How to set the optimizer goal:– Change the database configuration file (init.ora).

OPTIMIZER_MODE=FIRST_ROWS– Change settings for your session in SQL*Plus.

ALTER SESSION SET OPTIMIZER_GOAL=RULE;– “Influence” the optimizer with “hints”.

• Example, use rule based optimizer:SELECT /*+ RULE */ *FROM EMPLOYEEWHERE SALARY > 100000;

• Example, use full table scan:select /*+ FULL(E) FULL(D) */

e.employee_id, e.surname, e.firstnamefrom employee e,

department dwhere d.location=‘Indianapolis’

and d.dept_id = e.dept_id;

• Example, recommend an index:select /*+ index(E, emp_dept_idx) */

e.employee_id, e.surname, e.firstnamefrom employee e,

department dwhere d.location=‘Indianapolis’

and d.dept_id = e.dept_id;

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SQL Tuning

• Tracing Oracle SQL:– EXPLAIN PLAN, shows the execution plan.– SQL_TRACE, generates a trace file containing SQL executed by

your session and resources used.– tkprof, formats SQL_TRACE output.– AUTOTRACE, SQL*Plus command to show exuction plans and

statistics in your SQL*Plus session.– 3rd Party Tools. Numerous GUI tools to quickly show this

information, usually expensive.

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EXPLAIN PLANExecuting:

explain plan forselect /*+RULE */ e.surname, e.firstname, e.date_of_birth from employee e, customers c where e.surname=c.contact_surname and e.firstname=c.contact_firstname and e.date_of_birth=c.date_of_birth order by e.surname, e.firstname;

Formatting Plan Table in a Query:select rtrim(lpad(‘ ‘,2*level)|| rtrim(operation)||‘ ‘|| rtrim(options)||‘ ‘|| object_name) as query_plan from plan_table connect by prior id=parent_id start with id=0;

Execution Plan Query:query_plan----------------------------------------SELECT STATEMENT SORT ORDER BY NETED LOOPS TABLE ACCESS FULL CUSTOMERS TABLE ACCESS BY ROWID EMPLOYEES INDEX RANGE SCAN EMP_BOTH_IDX

SQL Tuning

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AcknowledgementsAcknowledgements

Loney, Oracle Database 10g The Complete ReferenceLoney, Oracle Database 10g The Complete Reference