Portal | Manuals | References | Downloads | Info | Programs | JCLs | Master the Mainframes
IBM Mainframe Computers Forums Index
 
Register
 
IBM Mainframe Computers Forums Index Mainframe: Search IBM Mainframe Forum: FAQ Memberlist Usergroups Profile Log in to check your private messages Log in
 

 

Problem about readprev command

 
Post new topic   Reply to topic    IBMMAINFRAMES.com Support Forums -> Mainframe Interview Questions
View previous topic :: :: View next topic  
Author Message
weibiao

New User


Joined: 08 Jun 2008
Posts: 5
Location: China

PostPosted: Fri Dec 24, 2010 1:39 pm    Post subject: Problem about readprev command
Reply with quote

In the index CI, there are two kinds of pointer: vertical and horizontal
pointer. The horizontal pointer point to the next same level index record.
When start browse a KSDS file, the horizontal pointer is very efficient to find the next SSI for readnext command. But when use readprev command to read file in descending order, there isn't any pointer that point to the previous SSI in the index CI.

So anyone know how readprev work? Return to the higher index level to find the previous SSI?

Thanks a lot!
Back to top
View user's profile Send private message

Robert Sample

Global Moderator


Joined: 06 Jun 2008
Posts: 7931
Location: Bellevue, IA

PostPosted: Sun Dec 26, 2010 7:09 am    Post subject:
Reply with quote

And if the previous key is located in the same data CI, what do you think will happen?

You are attempting to WAY oversimplify a very complex subject. And a subject that no one, other than IBM employees, are likely to be able to precisely answer. Why does it matter to you?
Back to top
View user's profile Send private message
weibiao

New User


Joined: 08 Jun 2008
Posts: 5
Location: China

PostPosted: Sun Dec 26, 2010 1:17 pm    Post subject:
Reply with quote

It's easy to get the previous key that is located in the same data/index CI. But can not find out any answer from the VSAM redbook for VSAM to get the previous key the located in different index CI.
One that I can assume is that CICS will put the higher index CI in a queue or stack, when readprev would read the previous key that located in previous SSI, it will read from the queue or stack to get the higher index CI, which will have a pointer pointed to the previous SSI.

Is there any IBM employees in this forum that can answer my question? Thanks!
Back to top
View user's profile Send private message
weibiao

New User


Joined: 08 Jun 2008
Posts: 5
Location: China

PostPosted: Mon Dec 27, 2010 7:24 am    Post subject:
Reply with quote

Hi Peter, thanks a lot.
I have read this book before. But can not found any answer.
Back to top
View user's profile Send private message
dick scherrer

Site Director


Joined: 23 Nov 2006
Posts: 19270
Location: Inside the Matrix

PostPosted: Mon Dec 27, 2010 10:13 am    Post subject:
Reply with quote

Hello,


If you want an answer from IBM, you should send a request to IBM.

Your technical support people will have the contact info.
Back to top
View user's profile Send private message
PeterHolland

Global Moderator


Joined: 27 Oct 2009
Posts: 2422
Location: Netherlands, Amstelveen

PostPosted: Mon Dec 27, 2010 5:06 pm    Post subject:
Reply with quote

VSAM uses an inverted index (B+ tree), depending on the number of index buffers that tree will be loaded in memory for fast searching. Reverse searching is probably done by using sideways links.

There is a lot of literature about B+ trees to get an idea how searching
is accomplished.

Im not sure if IBM is going to tell you about the inner workings of their
software. Beside that why bother? I didnt see any problem in your post
about readprev, only the fact that you wanted to know how something
is done.
Back to top
View user's profile Send private message
View previous topic :: :: View next topic  
Post new topic   Reply to topic    IBMMAINFRAMES.com Support Forums -> Mainframe Interview Questions All times are GMT + 6 Hours
Page 1 of 1

 

Search our Forum:

Similar Topics
Topic Author Forum Replies Posted
No new posts SYMNAMES problem jacobdng DFSORT/ICETOOL 7 Thu Dec 22, 2016 7:47 am
No new posts What is the command to check MODE of ... rohanthengal CLIST & REXX 7 Fri Nov 18, 2016 1:48 pm
No new posts Problem in writing Output file vickey_dw COBOL Programming 5 Mon Nov 14, 2016 11:14 pm
No new posts Problem in Running Query via JCL vickey_dw DB2 3 Tue Oct 18, 2016 11:11 pm
No new posts SDSF Command Avtrix CLIST & REXX 4 Fri Sep 30, 2016 11:13 am


Facebook
Back to Top
 
Mainframe Wiki | Forum Rules | Bookmarks | Subscriptions | FAQ | Tutorials | Contact Us