The following is taken from the Oracle Technology Network website. Some of the information may be specific to release 8.1.5 of Oracle, although DBMS_OUTPUT is also included with other releases.
The DBMS_OUTPUT package enables you to send messages from stored procedures, packages, and triggers.
The PUT and PUT_LINE procedures 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.
If you do not call GET_LINE, or if you do not display the messages on your screen in SQL*Plus or Enterprise Manager, then the buffered messages are ignored. The DBMS_OUTPUT package is especially useful for displaying PL/SQL debugging information.
Note: Messages sent using DBMS_OUTPUT are not actually sent until the sending subprogram or trigger completes. There is no mechanism to flush output during the execution of a procedure.
A trigger might want to print out some debugging information. To do this, the trigger would do:
DBMS_OUTPUT.PUT_LINE('I got here:'ªª:new.colªª' is the new value');
If you have enabled the DBMS_OUTPUT package, then 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), get the line of information back. For example:
BEGIN DBMS_OUTPUT.GET_LINE(:buffer, :status); END;
It could then display the buffer on the screen. You repeat calls to GET_LINE until status comes back as non-zero. For better performance, you should use calls to GET_LINES which can return an array of lines.
Enterprise Manager and SQL*Plus implement a SET SERVEROUTPUT ON command to know whether to make calls to GET_LINE(S) after issuing INSERT, UPDATE, DELETE or anonymous PL/SQL calls (these are the only ones that can cause triggers or stored procedures to be executed).
Summary of Subprograms
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 enabled.
Note: It is not necessary to call this procedure when you use the SERVEROUTPUT option of Enterprise Manager or SQL*Plus.
If there are multiple calls to ENABLE, then buffer_size is the largest of the values specified. The maximum size is 1,000,000, and the minimum is 2,000.
DBMS_OUTPUT.ENABLE ( buffer_size IN INTEGER DEFAULT 20000);
Parameters buffer_size Amount of information, in bytes, to buffer.
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 ENABLE, you do not need to call this procedure if you are using the SERVEROUTPUT option of Enterprise Manager or SQL*Plus.
PUT and PUT_LINE procedures
You can either place an entire line of information into the buffer by calling PUT_LINE, or you can build a line of information piece by piece by making multiple calls to PUT. Both of these procedures are overloaded to accept items of type VARCHAR2, NUMBER, or DATE to place in the buffer.
All items are converted to VARCHAR2 as they are retrieved. If you pass an item of type NUMBER or DATE, then when that item is retrieved, it is formatted with TO_CHAR using the default format. If you want to use a different format, then you should pass in the item as VARCHAR2 and format it explicitly.
When you call PUT_LINE, the item that 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 line exceeds the buffer limit, then you receive an error message.
Note: 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, Enterprise Manager or SQL*Plus do 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 SERVER OUTPUT ON SQL> BEGIN 2 DBMS_OUTPUT.PUT_LINE ('hello'); 3 DBMS_LOCK.SLEEP (10); 4 END;
DBMS_OUTPUT.PUT (item IN NUMBER); DBMS_OUTPUT.PUT (item IN VARCHAR2); DBMS_OUTPUT.PUT (item IN DATE); DBMS_OUTPUT.PUT_LINE (item IN NUMBER); DBMS_OUTPUT.PUT_LINE (item IN VARCHAR2); DBMS_OUTPUT.PUT_LINE (item IN DATE); DBMS_OUTPUT.NEW_LINE;
Parameters a Item to buffer.
This procedure puts an end-of-line marker. GET_LINE(S) returns "lines" as delimited by "newlines". Every call to PUT_LINE or NEW_LINE generates a line that is returned by GET_LINE(S).
GET_LINE and GET_LINES procedures
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 Enterprise Manager or 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.
DBMS_OUTPUT.GET_LINE ( line OUT VARCHAR2, status OUT INTEGER);
Parameters line Returns a single line of buffered information, excluding a final newline character: The maximum length is 255 bytes. status If the call completes successfully, then the status returns as 0. If there are no more lines in the buffer, then the status is 1.
DBMS_OUTPUT.GET_LINES ( lines OUT CHARARR, numlines IN OUT INTEGER);
CHARARR is a table of VARCHAR2(255).
Parameters lines Returns an array of lines of buffered information. The maximum length of each line in the array is 255 bytes. numlines Number of lines you want to retrieve from the buffer. After retrieving the specified number of lines, the procedure returns the number of lines actually retrieved. If this number is less than the number of lines requested, then there are no more lines in the buffer.
The DBMS_OUTPUT package is commonly used to debug stored procedures and triggers, as shown in Example 1. This package can also be used to enable you to retrieve information about an object and format this output, as shown in Example 2.
This is an example of a function that 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 = ' ªª counter ªª '; Wages = 'ªª TO_CHAR(total_wages)); /* Debug line */ counter := counter + 1; /* Increment debug counter */ END LOOP; /* Debug line */ DBMS_OUTPUT.PUT_LINE('Total wages = ' ªª TO_CHAR(total_wages)); RETURN total_wages;
Assume the user executes the following statements in the Enterprise Manager SQL Worksheet input pane:
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.
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;
-- 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 ' ªª statement_id ªª ', estimated cost = ' ªª line.position); END IF;