68 DBMS_OUTPUT

Using DBMS_OUTPUT

This section contains topics which relate to using the DBMS_OUTPUT package.

• Overview

• Security Model

• Operational Notes

• Exceptions

• Rules and Limits

• Examples

Overview

The package is typically used for debugging, or for displaying messages and reports to SQL*DBA or SQL*Plus (such as are produced by applying the SQL command DESCRIBE to procedures).

The PUT Procedure and PUT_LINE Procedure in this package enable you to place information in a buffer that can be read by another trigger, procedure, or package. In a separate PL/SQL procedure or anonymous block, you can display the buffered information by calling the GET_LINE Procedure and GET_LINES Procedure.

If the package is disabled, all calls to subprograms are ignored. In this way, you can design your application so that subprograms are available only when a client is able to process the information.

Security Model

The dbmsotpt.sql script must be run as user SYS. This creates the public synonym DBMS_OUTPUT, and EXECUTE permission on this package is granted to public.

Operational Notes

• If you do not call GET_LINE, or if you do not display the messages on your screen in SQL*Plus, the buffered messages are ignored.

• SQL*Plus calls GET_LINES after issuing a SQL statement or anonymous PL/SQL calls.

• Typing SET SERVEROUTPUT ON in SQL*Plus has the effect of invoking

  DBMS_OUTPUT.ENABLE (buffer_size => NULL);
  

  with no limit on the output.

• You should generally avoid having application code invoke either the DISABLE Procedure or ENABLE Procedure because this could subvert the attempt of an external tool like SQL*Plus to control whether or not to display output.

Exceptions

DBMS_OUTPUT subprograms raise the application error ORA-20000, and the output procedures can return the following errors:

Rules and Limits

• The maximum line size is 32767 bytes.

• The default buffer size is 20000 bytes. The minimum size is 2000 bytes and the maximum is unlimited.

Examples

Example 1: Using a Trigger to Produce Output

You can use a trigger to print out some output from the debugging process. For example, you could code the trigger to invoke:

DBMS_OUTPUT.PUT_LINE('I got here:'||:new.col||' is the new value'); 

If you have enabled the DBMS_OUTPUT package, then the text produced by this PUT_LINE would be buffered, and you could, after executing the statement (presumably some INSERT, DELETE, or UPDATE that caused the trigger to fire), retrieve the line of information. For example:

BEGIN 
   DBMS_OUTPUT.GET_LINE(:buffer, :status); 
END; 

You could then optionally display the buffer on the screen. You repeat calls to GET_LINE until status comes back as nonzero. For better performance, you should use calls to GET_LINES Procedure which can return an array of lines.

Example 2: Debugging Stored Procedures and Triggers

The DBMS_OUTPUT package is commonly used to debug stored procedures and triggers. This package can also be used to enable you to retrieve information about an object and format this output, as shown in "Example 3: Retrieving Information About an Object".

This function queries the employee table and returns the total salary for a specified department. The function includes several calls to the PUT_LINE procedure:

CREATE FUNCTION dept_salary (dnum NUMBER) RETURN NUMBER IS
   CURSOR emp_cursor IS
      SELECT sal, comm FROM emp WHERE deptno = dnum;
   total_wages    NUMBER(11, 2) := 0;
   counter        NUMBER(10) := 1;
BEGIN

   FOR emp_record IN emp_cursor LOOP
      emp_record.comm := NVL(emp_record.comm, 0);
      total_wages := total_wages + emp_record.sal
         + emp_record.comm;
      DBMS_OUTPUT.PUT_LINE('Loop number = ' || counter || 
         '; Wages = '|| TO_CHAR(total_wages));  /* Debug line */
      counter := counter + 1; /* Increment debug counter */
   END LOOP;
   /* Debug line */
   DBMS_OUTPUT.PUT_LINE('Total wages = ' ||
     TO_CHAR(total_wages)); 
   RETURN total_wages;

END dept_salary;

Assume the EMP table contains the following rows:

EMPNO          SAL     COMM     DEPT
-----        ------- -------- -------
1002           1500      500      20
1203           1000               30
1289           1000               10
1347           1000      250      20

Assume the user executes the following statements in SQL*Plus:

SET SERVEROUTPUT ON
VARIABLE salary NUMBER;
EXECUTE :salary := dept_salary(20);

The user would then see the following information displayed in the output pane:

Loop number = 1; Wages = 2000
Loop number = 2; Wages = 3250
Total wages = 3250

PL/SQL procedure successfully executed.

Example 3: Retrieving Information About an Object

In this example, the user has used the EXPLAIN PLAN command to retrieve information about the execution plan for a statement and has stored it in PLAN_TABLE. The user has also assigned a statement ID to this statement. The example EXPLAIN_OUT procedure retrieves the information from this table and formats the output in a nested manner that more closely depicts the order of steps undergone in processing the SQL statement.

/****************************************************************/
/* Create EXPLAIN_OUT procedure. User must pass STATEMENT_ID to */
/* to procedure, to uniquely identify statement.                */
/****************************************************************/
CREATE OR REPLACE PROCEDURE explain_out 
   (statement_id IN VARCHAR2) AS 

   -- Retrieve information from PLAN_TABLE into cursor EXPLAIN_ROWS.

   CURSOR explain_rows IS 
      SELECT level, id, position, operation, options,
         object_name 
      FROM plan_table 
      WHERE statement_id = explain_out.statement_id 
      CONNECT BY PRIOR id = parent_id 
         AND statement_id = explain_out.statement_id 
      START WITH id = 0
       ORDER BY id; 
 
BEGIN 

   -- Loop through information retrieved from PLAN_TABLE:

   FOR line IN explain_rows LOOP 

      -- At start of output, include heading with estimated cost.

      IF line.id = 0 THEN 
         DBMS_OUTPUT.PUT_LINE ('Plan for statement '
            || statement_id
            || ', estimated cost = ' || line.position); 
      END IF; 

      -- Output formatted information. LEVEL determines indention level.

      DBMS_OUTPUT.PUT_LINE (lpad(' ',2*(line.level-1)) ||
         line.operation || ' ' || line.options || ' ' || 
         line.object_name); 
   END LOOP; 

END; 

Data Structures

The DBMS_OUTPUT package declares 2 collection types for use with the GET_LINES Procedure.

TABLE Types

CHARARR Table Type

OBJECT Types

DBMSOUTPUT_LINESARRAY Object Type

CHARARR Table Type

This package type is to be used with the GET_LINES Procedure to obtain text submitted through the PUT Procedure and PUT_LINE Procedure.

Syntax

TYPE CHARARR IS TABLE OF VARCHAR2(32767) INDEX BY BINARY_INTEGER;

DBMSOUTPUT_LINESARRAY Object Type

This package type is to be used with the GET_LINES Procedure to obtain text submitted through the PUT Procedure and PUT_LINE Procedure.

Syntax

TYPE DBMSOUTPUT_LINESARRAY IS
     VARRAY(2147483647) OF VARCHAR2(32767);

Summary of DBMS_OUTPUT Subprograms

DISABLE Procedure

This procedure disables calls to PUT, PUT_LINE, NEW_LINE, GET_LINE, and GET_LINES, and purges the buffer of any remaining information.

As with the ENABLE Procedure, you do not need to call this procedure if you are using the SERVEROUTPUT option of SQL*Plus.

Syntax

DBMS_OUTPUT.DISABLE;

Pragmas

pragma restrict_references(disable,WNDS,RNDS);

ENABLE Procedure

This procedure enables calls to PUT, PUT_LINE, NEW_LINE, GET_LINE, and GET_LINES. Calls to these procedures are ignored if the DBMS_OUTPUT package is not activated.

Syntax

DBMS_OUTPUT.ENABLE (
   buffer_size IN INTEGER DEFAULT 20000);

Pragmas

pragma restrict_references(enable,WNDS,RNDS);

Parameters

Usage Notes

• It is not necessary to call this procedure when you use the SET SERVEROUTPUT option of SQL*Plus.

• If there are multiple calls to ENABLE, then buffer_size is the last of the values specified. The maximum size is 1,000,000, and the minimum is 2,000 when the user specifies buffer_size (NOT NULL).

• NULL is expected to be the usual choice. The default is 20,000 for backwards compatibility with earlier database versions that did not support unlimited buffering.

GET_LINE Procedure

This procedure retrieves a single line of buffered information.

Syntax

DBMS_OUTPUT.GET_LINE (
   line    OUT VARCHAR2,
   status  OUT INTEGER);

Parameters

Usage Notes

• You can choose to retrieve from the buffer a single line or an array of lines. Call the GET_LINE procedure to retrieve a single line of buffered information. To reduce the number of calls to the server, call the GET_LINES procedure to retrieve an array of lines from the buffer.

• You can choose to automatically display this information if you are using SQL*Plus by using the special SET SERVEROUTPUT ON command.

• After calling GET_LINE or GET_LINES, any lines not retrieved before the next call to PUT, PUT_LINE, or NEW_LINE are discarded to avoid confusing them with the next message.

GET_LINES Procedure

This procedure retrieves an array of lines from the buffer.

Syntax

DBMS_OUTPUT.GET_LINES (
   lines       OUT     CHARARR,
   numlines    IN OUT  INTEGER);

DBMS_OUTPUT.GET_LINES (
   lines       OUT     DBMSOUTPUT_LINESARRAY,
   numlines    IN OUT INTEGER);

Parameters

Usage Notes

• You can choose to retrieve from the buffer a single line or an array of lines. Call the GET_LINE procedure to retrieve a single line of buffered information. To reduce the number of calls to the server, call the GET_LINES procedure to retrieve an array of lines from the buffer.

• You can choose to automatically display this information if you are using SQL*Plus by using the special SET SERVEROUTPUT ON command.

• After calling GET_LINE or GET_LINES, any lines not retrieved before the next call to PUT, PUT_LINE, or NEW_LINE are discarded to avoid confusing them with the next message.

NEW_LINE Procedure

This procedure puts an end-of-line marker. The GET_LINE Procedure and the GET_LINES Procedure return "lines" as delimited by "newlines". Every call to the PUT_LINE Procedure or NEW_LINE Procedure generates a line that is returned by GET_LINE(S).

Syntax

DBMS_OUTPUT.NEW_LINE;

PUT Procedure

This procedure places a partial line in the buffer.

Syntax

DBMS_OUTPUT.PUT (
    item IN VARCHAR2);

Parameters

Exceptions

Usage Notes

• You can build a line of information piece by piece by making multiple calls to PUT, or place an entire line of information into the buffer by calling PUT_LINE.

• When you call PUT_LINE the item you specify is automatically followed by an end-of-line marker. If you make calls to PUT to build a line, then you must add your own end-of-line marker by calling NEW_LINE. GET_LINE and GET_LINES do not return lines that have not been terminated with a newline character.

• If your lines exceed the line limit, you receive an error message.

• Output that you create using PUT or PUT_LINE is buffered. The output cannot be retrieved until the PL/SQL program unit from which it was buffered returns to its caller.

  For example, SQL*Plus does not display DBMS_OUTPUT messages until the PL/SQL program completes. There is no mechanism for flushing the DBMS_OUTPUT buffers within the PL/SQL program.

  SQL> SET SERVEROUTPUT ON
  SQL> BEGIN
    2  DBMS_OUTPUT.PUT_LINE ('hello');
    3  DBMS_LOCK.SLEEP (10);
    4  END;
  

PUT_LINE Procedure

This procedure places a line in the buffer.

Syntax

DBMS_OUTPUT.PUT_LINE (
   item IN VARCHAR2);

Parameters

Exceptions

Usage Notes

• You can build a line of information piece by piece by making multiple calls to PUT, or place an entire line of information into the buffer by calling PUT_LINE.

• When you call PUT_LINE the item you specify is automatically followed by an end-of-line marker. If you make calls to PUT to build a line, then you must add your own end-of-line marker by calling NEW_LINE. GET_LINE and GET_LINES do not return lines that have not been terminated with a newline character.

• If your lines exceeds the line limit, you receive an error message.

• Output that you create using PUT or PUT_LINE is buffered. The output cannot be retrieved until the PL/SQL program unit from which it was buffered returns to its caller.

  For example, SQL*Plus does not display DBMS_OUTPUT messages until the PL/SQL program completes. There is no mechanism for flushing the DBMS_OUTPUT buffers within the PL/SQL program. For example:

  SQL> SET SERVEROUTPUT ON
  SQL> BEGIN
     2 DBMS_OUTPUT.PUT_LINE ('hello');
     3 DBMS_LOCK.SLEEP (10);
     4 END;