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Performance Tuning Enhancements in Oracle Database 10g

Oracle 10g includes many performance tuning enhancements including:

Automatic Performance Diagnostic and Tuning Features

Oracle 10g includes several features related to automatic performance diagnostics and tuning.

Most of these features are beyond the scope of this article and as such will be dealt with in separate articles.

Automatic Shared Memory Management

Automatic Shared Memory Management puts Oracle in control of allocating memory within the SGA. The SGA_TARGET parameter sets the amount of memory available to the SGA. This parameter can be altered dynamically up to a maximum of the SGA_MAX_SIZE parameter value. Provided the STATISTICS_LEVEL is set to TYPICAL or ALL and the SGA_TARGET is set to a value other than "0" Oracle will control the memory pools which would otherwise be controlled by the following parameters.

If these parameters are set to a non-zero value they represent the minimum size for the pool. These minimum values may be necessary if you experience application errors when certain pool sizes drop below a specific threshold.

The following parameters must be set manually and take memory from the quota allocated by the SGA_TARGET parameter.

Wait Model Improvements

A number of views have been updated and added to improve the wait model. The updated views include the following.

The new views include the following.

The following are some examples of how these updates can be used.

The V$EVENT_NAME view has had three new columns added (WAIT_CLASS_ID, WAIT_CLASS# and WAIT_CLASS) which indicate the class of the event. This allows easier aggregation of event details.

-- Display time waited for each wait class.
SELECT a.wait_class, sum(b.time_waited)/1000000 time_waited
FROM   v$event_name a
       JOIN v$system_event b ON a.name = b.event
GROUP BY a.wait_class;

WAIT_CLASS                  TIME_WAITED
--------------------------- -----------
Application                     .013388
Commit                          .003503
Concurrency                     .009891
Configuration                   .003489
Idle                         232.470445
Network                         .000432
Other                           .025698
System I/O                      .095651
User I/O                        .109552

9 rows selected.

The V$SESSION view has had several columns added that include blocking session and wait information. The wait information means it's no longer necessary to join to V$SESSION_WAIT to get wait information for a session.

-- Display blocked session and their blocking session details.
SELECT sid, serial#, blocking_session_status, blocking_session
FROM   v$session
WHERE  blocking_session IS NOT NULL;

no rows selected

-- Display the resource or event the session is waiting for.
SELECT sid, serial#, event, (seconds_in_wait/1000000) seconds_in_wait
FROM   v$session

       SID    SERIAL# EVENT                               SECONDS_IN_WAIT
---------- ---------- ----------------------------------- ---------------
       131         20 SQL*Net message from client                 .000015
       133        501 wakeup time manager                         .000138
       134      28448 SQL*Net message to client                         0
       135          4 queue messages                              .000003
       137          8 SQL*Net message from client                 .000132
       167          1 rdbms ipc message                                 0
       168          1 rdbms ipc message                                 0
       169          1 rdbms ipc message                           .079485
       170          1 pmon timer                                  .092645

29 rows selected.

The V$SESSION_WAIT_CLASS view allows you to see the session wait information broken down by wait class for each session.

-- Display session wait information by wait class.
FROM   v$session_wait_class
WHERE  sid = 134;

---------- ---------- ------------- ----------- ------------------- ----------- -----------
       134      28448    4217450380           1 Application                   2           0
       134      28448    3875070507           4 Concurrency                   1           2
       134      28448    2723168908           6 Idle                         57      392127
       134      28448    2000153315           7 Network                      68           5

4 rows selected.

The V$SESSION_WAIT_HISTORY view shows historical wait information which allows you to identify issues after the session has ended.

Automatic Optimizer Statistics Collection

By default Oracle 10g automatically gathers optimizer statistics using a scheduled job called GATHER_STATS_JOB. By default this job runs within a maintenance windows between 10 P.M. to 6 A.M. week nights and all day on weekends. The job calls the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC internal procedure which gathers statistics for tables with either empty or stale statistics, similar to the DBMS_STATS.GATHER_DATABASE_STATS procedure using the GATHER AUTO option. The main difference is that the internal job prioritises the work such that tables most urgently requiring statistics updates are processed first.

In some cases automatically gathering statistics can cause problems. Highly volatile tables and load tables may have their statistics gathered when there is an unrepresentative number of rows present. These situations can be avoided by using one of two methods.

System statistics and statistics for fixed object, such as dynamic performance tables, are not gathered automatically.

The default behaviour of the procedures in the DBMS_STATS package, and therefore the automatic stats collection job, can be altered using the SET_DATABASSE_PREFS, SET_SCHEMA_PREFS and SET_TABLE_PREFS procedures.

Dynamic Sampling

Dynamic sampling enables the server to improve performance by:

Dynamic sampling is controlled by the OPTIMIZER_DYNAMIC_SAMPLING parameter which accepts values from "0" (off) to "10" (aggressive sampling) with a default value of "2". At compile-time Oracle determines if dynamic sampling would improve query performance. If so it issues recursive statements to estimate the necessary statistics. Dynamic sampling can be beneficial in the following ways.

In addition to the OPTIMIZER_DYNAMIC_SAMPLING system parameter the dynamic sampling level can be set using the DYNAMIC_SAMPLING optimizer hint for specific queries like the following.

SELECT /*+ dynamic_sampling(emp 10) */
       empno, ename, job, sal
FROM   emp
WHERE  deptno = 30;

The results of dynamic sampling are repeatable provided no rows are inserted, updated or deleted from the sampled table. The OPTIMIZER_FEATURES_ENABLE parameter will turns off dynamic sampling if it is set to a version earlier than 9.2.0.

CPU Costing

By default the cost model for the optimizer is now CPU+I/O, with the cost unit as time.

Optimizer Hints

New hints:

Updated hints:

Renamed hints:

Deprecated hints:

Rule Based Optimizer Obsolescence

The Rule Based Optimizer (RBO) is now obsolete in Oracle 10g. The functionality is still present but no new functionality has been included in it and it is no longer supported by Oracle. It is only present to provide backwards compatibility during the migration to the query optimizer (Cost Based Optimizer). The results of this obsolescence are as follows.

Tracing Enhancements

The Oracle Trace functionality has been removed from Oracle 10g. Instead the SQL Trace and TKPROF functionality should be used.

In multi-tier environments where statements are passed to different sessions by the application server it can become difficult to trace an individual process from start to finish. To solve this problem Oracle have introduced End to End Application Tracing which allows a client process to be identified via the client identifier rather than the typical session id. Each piece of trace information is linked to the following information.

End to end tracing can be managed via Enterprise Manager or a set of APIs and views. Here are some examples of how to enable and disable to various types of tracing.

  -- Enable/Disable Client Identifier Trace.
  DBMS_MONITOR.client_id_trace_enable (client_id => 'my_id');

  DBMS_MONITOR.client_id_trace_disable (client_id => 'my_id');

  -- Enable/Disable Service, Module and Action Trace (various overloads).
  DBMS_MONITOR.serv_mod_act_trace_enable (
    service_name  => 'my_service');

  DBMS_MONITOR.serv_mod_act_trace_enable (
    service_name  => 'my_service',
    module_name   => 'my_module');

  DBMS_MONITOR.serv_mod_act_trace_enable (
    service_name  => 'my_service',
    module_name   => 'my_module',
    action_name   => 'INSERT');

  DBMS_MONITOR.serv_mod_act_trace_disable (
    service_name  => 'my_service',
    module_name   => 'my_module',
    action_name   => 'INSERT');

  -- Enable/Disable Session Trace (various overloads).

  DBMS_MONITOR.session_trace_enable (
    session_id => 15,
    serial_num => 1234);

  DBMS_MONITOR.session_trace_disable (
    session_id => 15,
    serial_num => 1234);

Once the trace files are produced the trcsess command line utility can be used to filter out the relevant data from multiple files. The utility accepts the following parameters:

At lease one of the search criteria must be specified. If more than one is specified only trace that matches all the criteria is consolidated. Examples of trcsess usage are shown below.

# Search all files for this session.
trcsess output=session.trc session=144.2274

# Search the specified files for this client identifier.
trcsess output=client.trc client_id=my_id db10g_ora_198.trc db10g_ora_206.trc

# Search the specified files for this service, module and action combination.
trcsess output=client.trc service=my_service module=my_module action=INSERT db10g_ora_198.trc db10g_ora_206.trc

Once the consolidated trace file is produced it can be processed by the TKPROF utility like any other SQL Trace file.

By default statistics are gathered at the session level. The DBMS_MONITOR package allows this to be altered to follow the client identifier, service or combinations of the service, module and action.

  -- Enable/Disable Client Identifier Statistics.
  DBMS_MONITOR.client_id_stat_enable (client_id => 'my_id');

  DBMS_MONITOR.client_id_stat_disable (client_id => 'my_id');

  -- Enable/Disable Service, Module and Action Statistics (various overloads).
  DBMS_MONITOR.serv_mod_act_stat_enable (
    service_name  => 'my_service');

  DBMS_MONITOR.serv_mod_act_stat_enable (
    service_name  => 'my_service',
    module_name   => 'my_module');

  DBMS_MONITOR.serv_mod_act_stat_enable (
    service_name  => 'my_service',
    module_name   => 'my_module',
    action_name   => 'INSERT');

  DBMS_MONITOR.serv_mod_act_stat_disable (
    service_name  => 'my_service',
    module_name   => 'my_module',
    action_name   => 'INSERT');

The gathered statistics can be displayed using the following views.

SAMPLE Clause Enhancements

The SAMPLE clause allows a query to return a limited sample of data by specifying a percentage of rows or blocks to scan. This clause can now be present in complex queries.

-- Query 10% or rows.
SELECT e.empno, e.ename, d.dname
FROM   emp SAMPLE (10) e
       JOIN dept d ON e.deptno = d.deptno;

-- Query 10% of blocks.
SELECT e.empno, e.ename, d.dname
FROM   emp SAMPLE BLOCK (10) e
       JOIN dept d ON e.deptno = d.deptno;

Hash Partitioned Global Indexes

Support for hash partitioned global indexes has been added in Oracle 10g which can improve performance when a small number of leaf blocks are experiencing high levels of contention. The syntax for creating of a hash partitioned global index is shown below.

CREATE INDEX hgidx ON tab (c1,c2,c3) GLOBAL

For more information see:

Hope this helps. Regards Tim...

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