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Using TRACE and EXPLAIN PLAN

Oracle Database Tips by Donald Burleson

TRACE and EXPLAIN PLAN allow users to monitor execution statistics of SQL and PL/SQL and provides information about how Oracle will actually execute the plan.

Instance Wide trace

The DBA can set up instance wide tracing by setting the SQL_TRACE initialization parameter to TRUE, however this is not suggested since then each session will generate a trace in the location specified by the initialization parameter USER_DUMP_DEST that is named for the users session id. However, the DBA should set the undocumented parameter "_public_trace_files" to TRUE in the test and development environments to allow the developers to see their trace files (on UNIX, on WINDOWS this isn't required).

In order to make the trace files more useful the DBA should also set the parameter TIMED_STATISTICS to TRUE. However, the DBA should verify that the release of Oracle you are using doesn't have any bugs associated with TIMED_STATISTICS and SQL_TRACE as there have been some reported against various 8i versions.

Trace in a session

As long as a user has ALTER SESSION permission they can alter their own session to be traced. The following commands will turn on tracing in a user session:

ALTER SESSION SET TIMED_STATISTICS=TRUE;

ALTER SESSION SET SQL_TRACE=TRUE;

The user should be careful to turn off tracing when it is no longer needed. Logging off of a session automatically terminates session specific tracing.

Tracing Forms, Reports

Tracing in forms, reports and procedures can be turned on by use of the DBMS_SESSION.SET_SQL_TRACE command, however the TIMED_STATISTICS parameter must still be set manually or via a call using EXECUTE IMMEDIATE on versions prior to Oracle8i unless the DBMS_SYSTEM package has been loaded. In Oracle9i the DBMS_SYSTEM.SET_INT_PARAM_IN_SESSION package can be used to alter the settings of session alterable parameters.

Tracing another session

The DBA will usually be the one turning on and off tracing for other sessions, however, this is done using the DBMS_SYSTEM. SET_SQL_TRACE_IN_SESSION package, the oradebug program or using system events.
 
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