The SET SERVEROUTPUT ON command is a directive used in Oracle SQLPlus and Oracle SQL Developer, which are tools for interacting with Oracle databases. This command enables the display of output from PL/SQL code, such as the output from the DBMS_OUTPUT.PUT_LINE procedure, directly in the SQLPlus or SQL Developer console. This feature is particularly useful for debugging and displaying results from procedural code blocks, functions, procedures, and triggers.
Here’s an overview of how it works and how you can use it:
Purpose
The primary purpose of SET SERVEROUTPUT ON is to allow developers to see the output generated by their PL/SQL code. Without this setting enabled, messages sent using DBMS_OUTPUT.PUT_LINE would not be visible in the SQL*Plus or SQL Developer output window.
Usage
Enabling Output Display: To enable the server output, you would execute the command:
SET SERVEROUTPUT ON;
You can also limit the buffer size (in bytes) for messages. If you anticipate a large amount of output, you can specify a size limit like so:
SET SERVEROUTPUT ON SIZE 1000000;
This sets the buffer size to 1,000,000 bytes. The default buffer size is 2000 bytes in SQL*Plus and 20000 bytes in SQL Developer, though these values can change with different versions of the tools.
Disabling Output Display: To disable the output, you use:
SET SERVEROUTPUT OFF;
Considerations
Buffer Size: There’s a limit to how much output can be stored in the buffer. If your PL/SQL block generates more output than the buffer can hold, you might not see all your output. Adjusting the buffer size with the SIZE parameter can help, but keep in mind that excessively large buffers can impact performance.
Performance: While SET SERVEROUTPUT ON is useful for debugging and development, it can impact performance. For production environments, it’s common to disable server output or to ensure that PL/SQL blocks do not unnecessarily use DBMS_OUTPUT.PUT_LINE.
Example
Here’s a simple example of how you might use SET SERVEROUTPUT ON with a PL/SQL block:
SET SERVEROUTPUT ON;
BEGIN
DBMS_OUTPUT.PUT_LINE('Hello, World!');
END;
/
Running the above code in SQL*Plus or SQL Developer with server output enabled will display “Hello, World!” in the console.
Conclusion
SET SERVEROUTPUT ON is a handy feature for anyone working with Oracle’s database tools, providing immediate feedback from PL/SQL execution right in the console. It’s essential for debugging and for when you need to verify the behavior of your PL/SQL code through immediate outputs.