ABCs of z/OS System Programming Volume 3 - IBM Redbooks

Sequence set
The sequence set is the lowest level of index, and it directly points (through an RBA) to the
data CI in the CA of the data component. Each index logical record:
► Occupies one index CI.
► Maps one CI in the data component.
► Contains pointers and high key information for each data CI.
► Contains horizontal pointers from one sequence set CI to the next keyed sequence set CI.
These horizontal pointers are needed because of the possibility of splits, which make the
physical sequence different from the logical collating sequence by key.
Index set
The records in all levels of the index above the sequence set are called the index set. An entry
in an index set logical record consists of the highest possible key in an index record in the
next lower level, and a pointer to the beginning of that index record. The highest level of the
index always contains a single index CI.
The structure of VSAM prime indexes is built to create a single index record at the lowest level
of the index. If there is more than one sequence-set-level record, VSAM automatically builds
another index level.
Cluster
A cluster is the combination of the data component (data set) and the index component (data
set) for a KSDS. The cluster provides a way to treat index and data components as a single
component with its own name. Use of the word cluster instead of data set is recommended.
Alternate index (AIX)
The alternate index is a VSAM function that allows logical records of a KSDS or ESDS to be
accessed sequentially and directly by more than one key field. The cluster that has the data is
called the base cluster, then an AIX cluster is built from the base cluster. Alternate indexes
eliminate the need to store the same data in different sequences in multiple data sets for the
purposes of various applications. Each alternate index is a KSDS cluster consisting of an
index component and a data component.
The records in the AIX index component contain the alternate key and the RBA pointing to the
alternate index data component. The records in the AIX data component contain the alternate
key value itself and all the primary keys corresponding to the alternate key value (pointers to
data in the base cluster). The primary keys in the logical record are in ascending sequence
within an alternate index value.
Any field in the base cluster record can be used as an alternate key. It can also overlap the
primary key (in a KSDS), or any other alternate key. The same base cluster may have several
alternate indexes varying the alternate key. There may be more than one primary key value
per the same alternate key value. For example, the primary key might be an employee
number and the alternate key might be the department name; obviously, the same
department name may have several employee numbers.
AIX cluster is created with IDCAMS DEFINE ALTERNATEINDEX command, then it is populated by
the BLDINDEX command. Before a base cluster can be accessed through an alternate index, a
path must be defined. A path provides a way to gain access to the base data through a
specific alternate index. To define a path, use the DEFINE PATH command. The utility to issue
this command is discussed in 4.14, “Access method services (IDCAMS)” on page 129.
Sphere
A sphere is a VSAM cluster and its AIX associated clusters’ data sets.
Chapter 4. Storage management software 165