Tracing Statements
------------------
You can automatically get a report on the execution path used by the SQL
optimizer and the statement execution statistics. The report is generated
after successful SQL DML (Data Manipulation Language - that is, SELECT, DELETE,
UPDATE and INSERT) statements. It is useful for monitoring and tuning the
performance of these statements.
Controlling the Report
----------------------
You can control the report by setting the AUTOTRACE system variable.
SET AUTOTRACE OFF - No AUTOTRACE report is generated. This is the
default.
SET AUTOTRACE ON EXPLAIN - The AUTOTRACE report shows only the optimizer
execution path.
SET AUTOTRACE ON STATISTICS - The AUTOTRACE report shows only the SQL
statement execution statistics.
SET AUTOTRACE ON - The AUTOTRACE report includes both the
optimizer execution path and the SQL
statement execution statistics.
SET AUTOTRACE TRACEONLY - Like SET AUTOTRACE ON, but suppresses the
printing of the user's query output, if any.
To use this feature, you must have the PLUSTRACE role granted to you and a
PLAN_TABLE table created in your schema. For more information on the PLUSTRACE
role and PLAN_TABLE table, see the AUTOTRACE variable of the SET command in
Chapter 6 of the SQL*Plus Guide.
Execution Plan
--------------
The Execution Plan shows the SQL optimizer's query execution path.
Each line of the Execution Plan has a sequential line number. SQL*Plus also
displays the line number of the parent operation.
The Execution Plan consists of four columns displayed in the following order:
Column Name Description
------------------------------------------------------------------------
ID_PLUS_EXP Shows the line number of each execution step.
PARENT_ID_PLUS_EXP Shows the relationship between each step and its
parent. This column is useful for large reports.
PLAN_PLUS_EXP Shows each step of the report.
OBJECT_NODE_PLUS_EXP Shows the database links or parallel query servers
used.
The format of the columns may be altered with the COLUMN command. For example, to stop the PARENT_ID_PLUS_EXP column being displayed, enter:
SQL> COLUMN PARENT_ID_PLUS_EXP NOPRINT
The default formats can be found in the site profile (for example, glogin.sql).
The Execution Plan output is generated using the EXPLAIN PLAN command. For
information about interpreting the output of EXPLAIN PLAN, see the
Oracle7 Server Tuning guide.
The following is an example of tracing statements for performance statistics and
query execution path.
If the SQL buffer contains the following statement:
SQL> SELECT D.DNAME, E.ENAME, E.SAL, E.JOB
2 FROM EMP E, DEPT D
3 WHERE E.DEPTNO = D.DEPTNO
The statement can be automatically traced when it is run:
SQL> SET AUTOTRACE ON
SQL> /
DNAME ENAME SAL JOB
-------------- ---------- ---------- ---------
ACCOUNTING CLARK 2450 MANAGER
ACCOUNTING KING 5000 PRESIDENT
ACCOUNTING MILLER 1300 CLERK
RESEARCH SMITH 800 CLERK
RESEARCH ADAMS 1100 CLERK
RESEARCH FORD 3000 ANALYST
RESEARCH SCOTT 3000 ANALYST
RESEARCH JONES 2975 MANAGER
SALES ALLEN 1600 SALESMAN
SALES BLAKE 2850 MANAGER
SALES MARTIN 1250 SALESMAN
SALES JAMES 950 CLERK
SALES TURNER 1500 SALESMAN
SALES WARD 1250 SALESMAN
14 rows selected.
Execution Plan
-----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 MERGE JOIN
2 1 SORT (JOIN)
3 2 TABLE ACCESS (FULL) OF 'DEPT'
4 1 SORT (JOIN)
5 4 TABLE ACCESS (FULL) OF 'EMP'
Statistics
----------------------------------------------------------
148 recursive calls
4 db block gets
24 consistent gets
6 physical reads
43 redo size
591 bytes sent via SQL*Net to client
256 bytes received via SQL*Net from client
33 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
14 rows processed
Note: The output may vary depending on the version of the server to
which you are connected and the configuration of the server.
References
-----------------------------------------------------------
SQL*PLus User's Guide and Reference Release 3.3.
Autotrace documented on Page 6-77. Part No. A42562-1.
Oracle7 Server Tuning Release 7.3. Part No. A32537-1.
Explain Plan documented on Page 6-18.
