Oracle’s DBMS_OUTPUT package is a built-in PL/SQL package provided by Oracle Database that developers and DBAs commonly use for debugging purposes and to print messages or outputs from PL/SQL blocks, procedures, functions, packages, and triggers. The DBMS_OUTPUT package is especially useful during development and testing phases, where understanding the flow of execution and values of variables is crucial.
Key Components
The DBMS_OUTPUT package primarily consists of procedures that manage the output buffer and control the display of messages. Some of the main procedures and functions include:
DBMS_OUTPUT.ENABLE: This procedure activates the output mechanism and optionally sets the buffer size. If not explicitly enabled, no output will be captured or displayed. The buffer size parameter is optional, and if omitted, a default size is used.
DBMS_OUTPUT.DISABLE: This procedure disables the output. After calling DISABLE, the DBMS_OUTPUT package will not capture any output until ENABLE is called again.
DBMS_OUTPUT.PUT: This procedure places a string into the output buffer. It does not add a line break at the end of the string, allowing multiple PUT calls to append to the same line.
DBMS_OUTPUT.PUT_LINE: Similar to PUT, but it adds a line break after the string, effectively placing each call’s output on a new line.
DBMS_OUTPUT.GET_LINE: These procedures retrieve a line or multiple lines from the output buffer. DBMS_OUTPUT.GET_LINE retrieves a single line, while DBMS_OUTPUT.GET_LINES retrieves multiple lines into an array. These are typically used for reading back the buffered output within PL/SQL for further processing.
DBMS_OUTPUT.NEW_LINE: This procedure adds a line break to the output buffer, allowing you to insert blank lines for better readability.
Usage
To use DBMS_OUTPUT in your PL/SQL code, you first need to enable the output using DBMS_OUTPUT.ENABLE. You can then use PUT or PUT_LINE to add messages to the output buffer. Finally, to view the output, you would typically rely on your client tool (such as SQL*Plus, SQL Developer, or TOAD) to display the contents of the buffer after the execution of your PL/SQL block or script.
It’s important to note that the client tool must be configured to retrieve and display the DBMS_OUTPUT buffer. For example, in SQL*Plus, you would use the SET SERVEROUTPUT ON command to enable the display of the buffer’s contents.
Considerations
Buffer Size Limitations: The output buffer has a size limit (the default is typically 20,000 bytes but can be increased up to a limit, depending on the database version). If your output exceeds this limit, you may need to flush the buffer periodically or increase the buffer size using the ENABLE procedure.
Performance Impact: Excessive use of DBMS_OUTPUT can impact performance, especially with large volumes of output. It’s generally recommended to remove or comment out DBMS_OUTPUT calls from production code.
Visibility: Output generated by DBMS_OUTPUT is not immediately visible until the PL/SQL block completes execution and control returns to the client tool. This behavior makes it unsuitable for real-time monitoring or logging in long-running processes.
In summary, DBMS_OUTPUT is a valuable package for debugging and development purposes within Oracle Database environments. However, its use should be carefully managed, keeping in mind the performance implications and limitations.