Earlier versions of Oracle offered some PL/SQL trace capabilities, but Oracle8i Database introduced an API that allows you to easily specify and control the tracing of the execution of PL/SQL procedures, functions, and exceptions. The DBMS_TRACE built-in package provides programs to start and stop PL/SQL tracing in a session. When tracing is turned on, the engine collects data as the program executes. The data is then written out to the Oracle Server trace file.

In addition to DBMS_TRACE, you can take advantage of the built-in function, DBMS_UTILITY.FORMAT_CALL_STACK, to obtain the execution call stack at any point within your application.

The PL/SQL trace facility provides a trace file that shows you the specific steps executed by your code. The DBMS_PROFILER package (described later in this chapter) offers a much more comprehensive analysis of your application, including timing information and counts of the number of times a specific line was executed.

DBMS_UTILITY.FORMAT_CALL_STACK

This function returns the execution call stack (the sequence of program calls) down to the point at which you call the function. Here is an example of the formatting of this stack string:

  ----- PL/SQL Call Stack ----- -
  object   line    object
  handle   number  name

  88ce3f74   8  package
                STEVEN.VALIDATE_REQUEST
  88e49fc4   2  function STEVEN.COMPANY_TYPE
  88e49390   1  procedure
                STEVEN.CALC_NET_WORTH
  88e2bd20   1  anonymous block

One of the best places to use this function is within an exception handler, as in:

  EXCEPTION
     WHEN OTHERS
     THEN
       DBMS_OUTPUT.PUT_LINE (
         DBMS_UTILITY.FORMAT_CALL_STACK); 
  END;

Better yet, grab this information and write it to your log table, so that the support and debug teams can immediately see how you got to the point where the problem reared its ugly head.

There is, by the way, one big problem with the exception section above: if your call stack is deep, the formatted string will exceed 255 characters in length. Before Ora cle Database 10g Release 2, DBMS_OUTPUT.PUT_LINE would raise an exception in such cases. To avoid this problem, you might consider using Darko Egersdorfer’s callstack package, found in the callstack.pkg file on the book’s web site.

Installing DBMS_TRACE

This package may not have been installed automatically with the rest of the built-in packages. To determine whether DBMS_TRACE is present, connect to SYS (or another account with SYSDBA privileges) and execute this command:

  BEGIN DBMS_TRACE.CLEAR_PLSQL_TRACE; END;

If you see this error:

  PLS-00201: identifier 'DBMS_TRACE.CLEAR_PLSQL_TRACE' must be declared

then you must install the package. To do this, remain connected as SYS (or another account with SYSDBA privileges), and run the following files in the order specified:

  $ORACLE_HOME/rdbms/admin/dbmspbt.sql

  $ORACLE_HOME/rdbms/admin/prvtpbt.plb

DBMS_TRACE programs

The following programs are available in the DBMS_TRACE package:

SET_PLSQL_TRACE
   Starts PL/SQL tracing in the current session

CLEAR_PLSQL_TRACE
   Stops the dumping of trace data for that session

PLSQL_TRACE_VERSION
   Gets the major and minor version numbers of the
   DBMS_TRACE package

To trace execution of your PL/SQL code, you must first start the trace with a call to:

  DBMS_TRACE.SET_PLSQL_TRACE (trace_level INTEGER);

in your current session, where trace_level is one of the following values:

• Constants that determine which elements of your PL/SQL program will be traced:
  
    DBMS_TRACE.trace_all_calls   constant INTEGER := 1;
    DBMS_TRACE.trace_enabled_calls
                                 constant INTEGER := 2;
    DBMS_TRACE.trace_all_exceptions
                                 constant INTEGER := 4;
    DBMS_TRACE.trace_enabled_exceptions
                                 constant INTEGER := 8;
    DBMS_TRACE.trace_all_sql     constant INTEGER := 32;
    DBMS_TRACE.trace_enabled_sql
                                 constant INTEGER := 64;
    DBMS_TRACE.trace_all_lines   constant INTEGER := 128;
    DBMS_TRACE.trace_enabled_lines
                                 constant INTEGER := 256;
  
• Constants that control the tracing process:
  
    DBMS_TRACE.trace_stop        constant INTEGER := 16384;
    DBMS_TRACE.trace_pause       constant INTEGER := 4096;
    DBMS_TRACE.trace_resume      constant INTEGER := 8192;
    DBMS_TRACE.trace_limit       constant INTEGER := 16;

By combining the DBMS_TRACE constants, you can enable tracing of multiple PL/SQL language features simultaneously. Note that the
con stants that control the tracing behavior (such as DBMS_TRACE.trace_ pause) should not be used in combination with the other constants (such as DBMS_TRACE.trace_enabled_calls).

To turn on tracing from all programs executed in your session, issue this call:

  DBMS_TRACE.SET_PLSQL_TRACE (DBMS_TRACE.trace_all_calls);

To turn on tracing for all exceptions raised during the session, issue this call:

  DBMS_TRACE.SET_PLSQL_TRACE (DBMS_TRACE.trace_all_exceptions);

You then run your code. When you are done, you stop the trace session by calling:

  DBMS_TRACE.CLEAR_PLSQL_TRACE;

You can then examine the contents of the trace file. The names of these files are generated by Oracle; you will usually look at the modification dates to figure out which file to examine. The location of the trace files is discussed in the later section, “Format of collected data.”

Note that you cannot use PL/SQL tracing with the shared server (formerly known as the multithreaded server, or MTS).