Use of Prefix SYS before dbms.output()

Question

Hi all,
This may be silly. But i have a doubt what is the difference between the below two:
DBMS_OUTPUT.PUT_LINE('TEST');
AND
SYS.DBMS_OUTPUT.PUT_LINE('TEST');
What is the use of SYS prefix?
Regards
Vijayaragavan

Answer 1

788844 wrote:
This may be silly. But i have a doubt what is the difference between the below two:
DBMS_OUTPUT.PUT_LINE('TEST');
AND
SYS.DBMS_OUTPUT.PUT_LINE('TEST');
Not silly at all. This is a scope resolution issue.
All programming languages have scope resolution. You refer to a call (method, function, procedure, etc). The system (compiler or run-time engine) needs to figure out exactly what you are referencing , in order to execute that call.
In the case of PL/SQL, the PL/SQL engine will first use the local scope - in other words, the objects (procedure, functions, variables, etc) defined in the current code unit and schema.
If it fails to resolve the call reference, it looks at the public scope.
DBMS_OUTPUT.put_line() is a package call reference. So the 1st check would the local check. Does such a package exist in the current schema? Is there a local synonym that can be used?
If this fails, scope becomes public. Is there a public reference (synonym in this case) that resolves this?
The answer for DBMS_OUTPUT is yes - and that public synonym refers to the package owned by the SYS schema.
If you had a local package with the same name, or a local synonym, then that would have been used.
When you now code SYS.DBMS_OUTPUT.put_line() instead, you explicitly state the scope to the PL/SQL engine. You are telling it that you are referring to the DBMS_OUTPUT object owned by the SYS schema.
Both methods have certain advantages and disadvantages.
Simplistically, implicit scope (where you leave it up the engine to find the object you are referencing) provides more flexibility. Let's say you need to modify the behaviour of DBMS_OUTPUT - have it for example also write output to a log file. You can create your own DBMS_OUTPUT package with the exact same interface as the SYS.DBMS_OUTPUT package. Inside your custom package you can write output to a log file and then call the SYS.DBMS_OUTPUT package to do its part.
The original source code using DBMS_OUTPUT.put_line() works unchanged. All you have now introduced is a local reference that satisfy the scope - and your local DBMS_OUTPUT package is now used instead.
Explicit scope can be argued to be more secure. As no-one can "hijack" your code's call by manipulating scope resolution - and do the above and create a custom DBMS_OUTPUT package. But this also makes the code a lot more rigid.
In general in PL/SQL - implicit scope is desirable as it provides the flexibility that is often needed. When one writes "trusted" code (code that does superuser stuff and needs to be trusted to always do the right thing), then explicit scope should be considered instead.