Autotrace

autotrace

The autotrace utility is a very underutilized feature of SQL*Plus. It offers statement tracing and instant feedback on any successful SELECT, INSERT, UPDATE or DELETE statement. The utility requires a plan table (for the Explain Plan ) under the current user’s schema. In addition, it requires the plustrace or DBA role for the user executing the trace. The source for the PLUSTRACE role can be found in $ORACLE_HOME/sqlplus/admin/plustrce.sql.

Tracing is enabled via the set autotrace command in SQL*Plus:

SQL> set autotrace on

SQL> select ename from emp where empno = 1122;

no rows selected

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE

1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP'

2 1 INDEX (UNIQUE SCAN) OF 'PK_EMP' (UNIQUE)

Statistics

----------------------------------------------------------

83 recursive calls

0 db block gets

21 consistent gets

3 physical reads

0 redo size

221 bytes sent via SQL*Net to client

368 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

autotrace provides instantaneous feedback including the returned rows, execution plan and statistics. The user doesn’t need concerned about trace file locations and formatting since the output is displayed instantly on the screen. This is very important data that can be used to tune the SQL statement.

autotrace supports the following options:

· on – Enables all options.

· on explain – Displays returned rows and the explain plan.

· on statistics – Displays returned rows and statistics.

· trace explain – Displays the execution plan for a select statement without actually executing it. (set autotrace trace explain)

· traceonly – Displays execution plan and statistics without displaying the returned rows. This option should be used when a large result set is expected.

autotrace is so easy to use that it should be the first tracing utility used for most SQL performance tuning issues. tkprof can be used for more detailed analysis.