VSAM - Use of DEFINE PATH vs. DEFINE ALTERNATEINDEX

Abhi Nature · New User Joined: 14 Dec 2011 Posts: 17 Location: India

Hello,

'DEFINE ALTERNATEINDEX' creates and relates the alternate index to the cluster. So why is it necessary to 'DEFINE PATH' which does the same (relates)?

Thanks.

Bill Woodger · Posted: Wed Aug 19, 2015 5:16 pm

Perhaps you can provide links to the documentation which shows they are the same?

Abhi Nature · New User Joined: 14 Dec 2011 Posts: 17 Location: India

Syntax for 'DEFINE ALTERNATEINDEX'
DEFINE
ALTERNATEINDEX
(NAME(entryname)
RELATE(entryname)
.
.
Other parameters.
Creates and relates (to base cluster) the alternate index.

Syntax 'DEFINE PATH'
DEFINE
PATH
(NAME(entryname)
PATHENTRY(entryname)
.
.
Other parameters.

If 'DEFINE ALTERNATEINDEX' creates and relates the alternate index to the base cluster, what is the use of 'DEFINE PATH'?

Akatsukami · Posted: Wed Aug 19, 2015 5:43 pm

Perhaps you should read the fine manual and find out for yourself.

Abhi Nature · New User Joined: 14 Dec 2011 Posts: 17 Location: India

I did read the manual before posting the question. But I don't think it answers the question.

I do understand that 'DEFINE PATH' is there due a reason, but I was wondering why would a PATH be required if we have already related it to the cluster during creation of alternate index.

In my opinion, the RELATE parameter (while creating alternate index) should already create PATH.

Akatsukami · Posted: Wed Aug 19, 2015 6:09 pm

Robert Sample · Posted: Wed Aug 19, 2015 6:10 pm

The RELATE in the alternate index definition gives IDCAMS the base cluster data from which the data for the alternate index is read. Once the build of the alternate index is done, it exists INDEPENDENTLY of the base cluster -- the alternate index has a DATA component and an INDEX component just like the base cluster.

The DEFINE PATH establishes the connection between the alternate index and the base cluster. The DEFINE ALTERNATEINDEX does not do this -- it merely reads the data to build the index. If you refer to the alternate index with a DD statement, you get the alternate index -- and only the alternate index. If you refer to the PATH on the DD statement, the system opens both the alternate index AND the base cluster (since they are related via the PATH).

Abhi Nature · New User Joined: 14 Dec 2011 Posts: 17 Location: India

Okay. To conclude...
PATH is there due to one way in which VSAM was designed. There could have been another approach whereby PATH could have been avoided (like creating the path in first few bytes of the alternate index). But obviously PATH would be the most efficient way to use the alternate index (since IBM chose it).

Thanks everyone for your time and help.

Bill Woodger · Posted: Wed Aug 19, 2015 6:42 pm

enrico-sorichetti · Posted: Wed Aug 19, 2015 7:13 pm

for all You might want to know, see here
www.redbooks.ibm.com/abstracts/sg246105.html?Open

enrico-sorichetti · Posted: Wed Aug 19, 2015 7:28 pm

Abhi Nature · New User Joined: 14 Dec 2011 Posts: 17 Location: India

I agree that the important thing is what the parameter holds rather than what its name suggests. Instead of RELATE it could have been BASECLUSTER, but in either case 'DEFINE ALTERNATEINDEX' relates base cluster with its alternate index, or so it seems. If it does, then why 'DEFINE PATH' is needed, there was the confusion.

enrico-sorichetti · Posted: Wed Aug 19, 2015 9:41 pm

the current design is to have the AIX and the BASE CLUSTER processing unbundled as much as possible,

for example
I have had a few times the need to process the AIX by itself,
quite a few IBM and non-IBM products have the same need

if the design had been the way You are proposing there would be no way to do what I outlined

anyway read and meditate on the redbook for which I posted the link.

Pete Wilson · Posted: Tue Sep 08, 2015 3:13 am

It is simple and logical..

The Base cluster is defined with a KEY to access the records, e.g looking from position 0 for a length of 8. You cannot have more than one key specified for the base cluster.

The AIX is defined with a different KEY to the base cluster, e.g. from position 10 for a length of 6, but the RELATE to the base cluster means you're looking at the same records in the base cluster.

You then do a BUILDINDEX which copies all the AIX key parts of the records in the base cluster into the AIX.

You then define the PATH which is a different name to the Base cluster and relate it to the AIX using the PATHENTRY.

Now, when you specify the PATH name to access the Base cluster, it uses the KEYS in the AIX to access the records in the base cluster.