[bbk2301] 第44集 - Chapter 11-SQL Statement Tuning(03)

Generate the Execution Plan

• Can be used without tracing
• Needs the plan_table table utlxplan.sql
• Create the explain plan:

SQL> explain plan for
  2  select * from my_all_objects;

Explained.

Query plan_table Table

Query plan_table to display the execution plans:

• Query plan_table directly.
• Use script utlxpls.sql(hide Paralle Query information)
• Use script utlxplp.sql(show Parallel Query information)
• Use the dbms_xplan package.（recommend method）

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 4167016233

---------------------------------------------------------------------------------
| Id  | Operation         | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |             |    27 |   567 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| DEPARTMENTS |    27 |   567 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------

8 rows selected.

 

execution plan解读:先执行里面的TABLE ACCESS FULL，在执行外面的SELECT STATEMENT;看缩进，也就是说先执行缩进里面的，在执行往外的.如果同级别的，先执行上面的，在执行下面的.

SQL> @?/rdbms/admin/utlxpls.sql

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 4167016233

---------------------------------------------------------------------------------
| Id  | Operation         | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |             |    27 |   567 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| DEPARTMENTS |    27 |   567 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------

8 rows selected.

 

注意：在产生一个新的plan之前，最好把表中存在的数据给清空. (后期版本的oracle可能自动清空之前的plan)

推荐书籍<<Effective Oracle by Design>>

Using SQL Trace & TKPROF 

To use SQL trace and TKPROF

• Set the initialization parameters.

SQL>ALTER SESSION SET sql_trace = True;

 

• Run the application

SQL>ALTER SESSION SET sql_trace = False;

 

• Format the trace file with TKPROF
• Interpret the output. 

 一般情况下，只是session级别，不会是system级别；如果是system级别，会产生大量的文件，造成系统负担.

Enable & Disable SQL Trace

At he instance level:SQL_TRACE = {True|False}

At the session level:

SQL>ALTER SESSION SET SQL_TRACE = {True|False}
SQL>EXECUTE dbms_session.set_sql_trace({True|False});
SQL>EXECUTE dbms_system.set_sql_trace_in_session(session_id,serial_id,{True|False})

 

Formatting the Trace File

$tkprof tracefile.trc output.txt [options]

 

 

TKPROF Statistics

• Count:Number of execution calls
• CPU:CPU seconds used
• Elapsed:Total elapsed time
• Disk:Physical reads
• Query:Logical reads for consistent read
• Current:Logical reads in current mode
• Rows:Rows processed

 

SQL*Plus Autotrace

• Create the plan_table table.
• Create and grant the plustrace role.

SQL>@ORACLE_HOME/sqlplus/admin/plustrce.sql
SQL>GRANT plustrace TO scott;

 

• Autotract syntax:

SET AUTOTRACE [Off|On|Traceonly] 

　　　　　　　　   [Explain|statistics]

 

 

Summary

In this lesson,you should have learned how to:

• Describe how the  optimizer is used.
• Describe how hints are used
• Explain the concept of plan stability
• Explain the use of stored outlines
• Use SQL Trace and TKPROF