question and answers related to SAP BASIS

What is private mode? When does user switch to user mode?
Private mode is a mode where the heap data is getting exclusively allocated by the user and is no more shared across the system. This happens when your extended memory is exhausted.
  
What is osp$ mean? What if user is given with this authorisation?
OPS$ is the mechanism the <SID>adm users uses to connect to the database .
Why do you use DDIC user  not SAP* for Support Packs and SPam?
Do _NOT_ use neither DDIC nor SAP* for applying support packages. Copy DDIC to a separate user and use that user to apply them.
Can you kill a Job?
Yes - SM37 - select - kill
If you have a long running Job,  how do you analyse?
Use transaction SE30.
How to uncar car/sar files in a single shot?
on Unix: 
$ for i in *.SAR; do SAPCAR -xvf $i; done
When we should use Transactional RFC ?
A "transactional RFC" means, that either both parties agree that the data was correctly transfered - or not. There is no "half data transfer".
What is the use of Trusted system. I know that there is no need of UID and PWD to communicate with partner system. In what situation it is good to go for Trusted system ?
E. g. if you have an R/3 system and a BW system and don't want to maintain passwords. Same goes for CRM and a lot of other systems/applications.
Let me know if my understanding below is correct:
1) By default the RFC destination is synchronous
2) Asynchronous RFC is used incase if the system initiated the RFC call no need to wait for the response before it proceeds to something else.
Yes - that's right.
But keep in mind, that it's not only a technical issue whether to switch to asynchronous. The application must also be able to handle that correctly.
Which table contains the details related to Q defined in SPAM? Is there a way to revert back the Q defined? If yes, How?
There is a "delete" button when you define the queue. If you already started the import it's no more possible since the system will become inconsistent.
What is a developer key? and how to generate a developer key?
The developer key is a combination of you installation number, your license key (that you get from http://service.sap.com/licensekey) and the user name. You need this for each person that will make changes (Dictionary or programs) in the system.
What is XI3.0 ? EXPLAIN XI = Exchange Infrastructure - Part of Netweaver 2004.
SAP Exchange Infrastructure (SAP XI) is SAP's enterprise application integration (EAI) software, a component of the NetWeaver product group used to facilitate the exchange of information among a company's internal software and systems and those of external parties. Like other NetWeaver components, SAP XI is compatible with software products of other
companies. 
SAP calls XI an integration broker because it mediates between entities with varying requirements in terms of connectivity, format, and protocols. According to SAP, XI reduces integration costs by providing a common repository for interfaces. The central component of SAP XI is the SAP Integration Server, which facilitates interaction between diverse operating systems and applications across internal and external networked computer systems. 
How to see when were the optimizer stats last time run? We are using win2k, oracle 9, sapr346c.
Assumed DB=Oracle
   
Select any table lets take MARA here but you should do the same for MSEG and few others to see whether the dates match or not.Run the following command on the command prompt:-
   
  select last_analyzed from dba_tables where table_name like '%MARA%';