Oracle Performance Tuning

Eric Geng2/24/2011

Agenda

• Oracle Basics

• Performance Tuning Methods

• SQL Tuning

• Database

• Instance

• Multi-Processes, share memory based C program

• Interfaces• SQL, PL/SQL• JDBC, OCI, Pro*C etc

• Administration Interfaces• Sqlplus, RMAN• Enterprise Manger

Oracle Architecture Overview

Processes

• PMON – Process Monitor

• SMON – System Monitor

• DBWR – Data Base Writer

• LGWR – Log Writer

• CKPT – Checkpoint

• ARCH – Log Archive

Wait Event

• Over 800 wait events, classified to 10+ types.

• They are self-diagnostic instruments – lots of counts and timers.

• Events• db file scattered read• db file sequential read• enqueue waits • library cache latch/mutex• log file sync• buffer busy waits• free buffer waits• …

Dynamic Views

• System statistics• v$sys_time_model, v$sysstat

• Session statistics• v$session, v$sesstat

• Contention statistics• v$lock, v$latch

• SQL staticstics• v$sql, v$sql_plan, v$sql_plan_statistics_all

Statspack/AWR

• Capture dynamic statistics into snapshots, then summary database activities between two snapshots.

• Statspack(9i)• External scripts and packages• Store snapshots in own-created table• DBMS_STATSPACK• sp*.sql

• Automatic Workload Repository(10g)• Built-in feature: scripts, packages, tables, automatic job• Store snapshots in DBA_HIST_* tables• DBMS_WORKLOAD_REPOSITORY• awr*.sql

AWR report

Enterprise Manager

• ASH• Sampling in seconds, support real-time Performance view in OEM• Store in SGA, v$active_session_history• Can also generate reports

• ADDM• Automatic Database Diagnostic Monitor• Identify issues and give recommendations from snapshots

• SQL Tuning Advisor• Create sql profile for sql statement

• SQL Access Advisor• Advice on index etc for sql statement

Enterprise Manager

Top SQL

• Elapsed time

• Parse time

• Executions

• Buffer gets

• Physical reads

Basic terminology

• An optimizer is the part of the RDBMS responsible for determining the most efficient way for a SQL statement to access data

• An execution plan is the complete sequence of operations required to resolve a SQL statement (the combination of access paths and join methods). The in-memory structure of a execution plan also can be called a cursor.

• A join method determines how two tables will be joined together (only two tables can be joined together at a time)

Hard parse

• Syntax analysis• Analyze "text" of SQL query• Detect syntax errors• Create internal query representation

• Semantic Checking• Validate SQL statement• View analysis• Incorporate constraints, triggers, etc.

• Query Optimization• Modify query to improve performance (Query Rewrite)• Choose the most efficient "access plan" (Query Optimization)

• Plan Generation

Optimizer

• Optimizer mode• RULE, CHOOSE, FIRST_ROWS, ALL_ROWS

• RULE is not supported

• Cost Based Optimizer• Highly depends on statistics

• Init parameters can affect the CBO• optimizer_mode• optimizer_index_cost_adj• optimizer_index_cache• data_block_multiple_read_count

Execution Plan

• Explain plan for <query>• Select * from table(dbms_xplan.display);

• dbms_xplan.display

• dbms_xplan.display_cursor

• dbms_xplan.display_awr

• Query with /*+ gather_plan_statistics */• more details in v$sql_plan_statistics_all• dbms_xplan.display_cursor(null, null, ‘ALLSTAT LAST’) • useful to find out wrong estimation by optimizer

Execution Plan

Plan of ‘ALLSTATS’

How to read an execution plan

• Order• Inner -> outer• At same level, up -> down. Some operations are iterative.

• Rows• Estimated by optimizer. Often cause problem.

• Time• Estimated by optimizer. Very inaccurate.

• A-Rows | A-Time• Actual numbers. Collected by /*+ gather_plan_statistics */

Operations in execution plan

• TABLE ACCESS FULL

• INDEX RANGE SCAN / TABLE ACCESS BY INDEX ROWID

• NESTED LOOPS / HASH JOIN / MERGE JOIN

• MERGE JOIN CARTESIAN

• INLIST ITERATOR

• SORT

• AGGREGATE

• FILTER

SQL trace/tkprof

• Alter session set sql_trace = true (SQL*PLUS)

• Alter session set events '10046 trace name context forever, level 12‘• Level 1 equals sql_trace=true• Level 4 includes bind values• Level 8 includes wait time statistics• Level 12 includes both bind values and wait time stats.

• dbms_monitor.session_trace_enable(sid, serial#, waits, binds)

• Trace file is in USER_DUMP_DEST folder

• tkprof <trace_file> <output_file>

SQL trace

TKPROF

Autotrace

• Set autotrace on/traceonly (SQL*PLUS)

• Besides the plan –

10053 trace (optimizer trace)

• Alter session set events ’10053 trace name context forever, level 1‘

SQL Tuning

• Common rules• Filter as early as possible and as much as possible • Join order is more significant than join method

• Methods• Statistics• Hints (sql profile / outline)• Rewrite• Index• Schema

Statistics

• DBMS_STATS.GATHER_XXX_STATS• analyze table xxx compute/estimate statistics is deprecated• GATHER_STATS_JOB

• USER_TABLES• NUM_ROWS, BLOCKS, AVG_ROW_LEN

• USER_TAB_COL_STATISTICS• NUM_DISTINCT, NUM_NULLS, NUM_BUCKETS, DENSITY

• USER_INDEXES• NUM_ROWS, DISTINCT_KEYS, LEAF_BLOCKS, CLUSTERING_FACTOR,

BLEVEL, AVG_LEAF_BLOCKS_PER_KEY

• USER_HISTOGRAM• ENDPOINT_NUMBER, ENDPOINT_VALUE

Hint /*+ */

• Join order• ORDERED, LEADING

• Join method• USE_NL, USE_HASH, USE_MERGE, STAR

• Query transformation• USE_CONCAT, NO_EXPAND, UNNEST, PUSH_SUBQ

• Index• INDEX, INDEX_JOIN, INDEX_COMBINE

• Stats• DYNAMIC_SAMPLING, CADINALITY, SELECTIVITY

• Sql Profile is a set of hints fundamentally

SQL rewrite

• In / exists

• Not in / not exists

• Correlated / non-correlated

• Join / SubQuery

Challenges for stable plan

• Volatile data

• Screw data / histograms / bind value peeking

• Data correlation

Application Tuning

• Avoid unnecessary query• Check logics• Cache

• Avoid to fetch too many rows• Pagination

• Avoid complex query• Material views• Denormalize

• Optimize locking

• Optimize indexes

OLTP applications

• Use connection pool

• Use bind variables (CURSOR_SHARING=FORCE?)

• Prefer OPTIMIZER_MODE=FIRST_ROW

• Enforce pagination

• Caution with any query which join too many tables

• Caution with any Full Table Scan or Fast Full Index Scan on large table

• Caution with any Sort, Aggregation, Hash Join, Merge Join, especially Merge Join Cartesian on large sets

Links

• oracle-l@freelists.org• https://groups.google.com/forum/?pli=1#!forum/com

p.databases.oracle.server

• http://tahiti.oracle.com/ • http://www.oaktable.net/ • http://blogs.oracle.com/optimizer/ • http://blog.tanelpoder.com/• http://jonathanlewis.wordpress.com/• http://kevinclosson.wordpress.com/ • http://structureddata.org/ • http://www.eygle.com/

Q&A

Q & A