Oracle’s DBMS_OUTPUT package is a built-in PL/SQL package that provides a way for the PL/SQL blocks, subprograms, triggers, and packages to send output to a buffer that can be read by another session. This is especially useful during the development and debugging phases of PL/SQL program units. Among its various procedures, the ENABLE procedure is used to prepare the DBMS_OUTPUT buffer to capture output.
Syntax
The syntax for DBMS_OUTPUT.ENABLE is as follows:
DBMS_OUTPUT.ENABLE( buffer_size IN INTEGER DEFAULT 20000 );
buffer_size: This parameter specifies the maximum buffer size in bytes. Oracle allows you to specify a buffer size up to a limit (which can vary by Oracle version; in some versions, it could be up to 1,000,000 bytes). If the size is not specified, a default value is used (20,000 bytes in many versions). Specifying a value of NULL or not specifying a value at all uses the default buffer size. If you specify 0, it means there is no limit to the buffer size, but this option should be used with caution to avoid excessive memory usage.
How It Works
When enabled, the DBMS_OUTPUT buffer stores output generated by DBMS_OUTPUT.PUT_LINE and other procedures in the DBMS_OUTPUT package. This output can then be viewed using tools like SQL*Plus, SQL Developer, or TOAD, which can fetch and display the contents of the buffer after the execution of PL/SQL code. To view the output, the session that intends to read the buffer must execute the DBMS_OUTPUT.GET_LINE or DBMS_OUTPUT.GET_LINES procedure after the PL/SQL code execution.
Use Cases
Debugging: It’s a common practice to use DBMS_OUTPUT.PUT_LINE for debugging PL/SQL blocks by printing variable values or messages to indicate the execution flow.
Development: During development, programmers can use it to display the status of variables, intermediate results, or confirmation messages that procedures have been executed.
Important Considerations
The buffer has a limited size, and when the limit is reached, additional output is discarded without warning. This makes it essential to choose an appropriate buffer size based on the expected volume of output.
The output is not visible until the PL/SQL block or subprogram completes execution unless the tool or environment you use fetches the buffer contents periodically.
For long-running processes or those that generate a lot of output, consider writing the output to a persistent store (like a table) instead of relying on DBMS_OUTPUT, to avoid losing messages once the buffer limit is reached.
Remember to enable the serveroutput in your SQL client tool (e.g., in SQL*Plus, use SET SERVEROUTPUT ON) to view the output.
The DBMS_OUTPUT.ENABLE procedure is a powerful tool for debugging and development, but it should be used judiciously, keeping in mind its limitations and impact on performance and memory usage.