IMS DB Interview Questions (21 - 30)

21. Explain Index Pointer Segment.

An Index Pointer Segment is a special type of segment in IMS databases used to improve data retrieval performance. It points to the actual data segments, helping IMS find them quickly (like an index in a book). For example - If STUDENT-ID is indexed, IMS stores a pointer to where the STUDENT segment is located.

22. What is a Concatenated Segment?

When two or more segments are combined to form a unique key. For example - In a LIBRARY database, BOOK + AUTHOR could be concatenated to uniquely identify a book.

23. What is a Database Field?

In IMS, a Database Field is the smallest unit of data within a segment (like a column in a table). For example - A STUDENT segment may have fields like STUDENT-ID, NAME, AGE.

24. What are the Two Types of Fields Available in IMS DB?

IMS DB supports two main types of fields:

  • Key Fields (Sequence Fields): Used to sort and retrieve segments (e.g., STUDENT-ID).
  • Non-Key Fields: Hold extra data but are not used for searching (e.g., ADDRESS, PHONE).

25. What is Reorganization in IMS DB?

A maintenance process to optimize the database by:

  • Removing deleted segments.
  • Rebuilding indexes for faster access.
  • Freeing up unused space.

Why needed? Over time, databases get fragmented (like a messy closet). Reorg cleans it up!

26. What is SSA?

SSA stands for Segment Search Argument. It's a parameter used in IMS to specify criteria for locating specific segment occurrences within the database. By using SSAs, you can direct IMS to retrieve or manipulate only those segments that meet certain conditions. For example - Here, (STUDENT-ID = '1001') is the SSA.

27. What is the Function of SSA?

The primary function of an SSA is to qualify a DL/I (Data Language/I) call by specifying which segment occurrences to operate on. This qualification can be based on segment names, field values, or relational conditions. It helps IMS find the exact segment(s) you want to read/update/delete. It works like a WHERE clause in SQL.

28. What are Qualified and Unqualified Calls?

  • Unqualified Calls: No SSA is used → retrieves the first segment of the specified type. For example - GU STUDENT (gets the first student)
  • Qualified Calls: Uses an SSA to filter segments. For example - STUDENT-ID = '1001' (gets only student with ID 1001)

29. What are Qualified & Unqualified SSAs?

  • Unqualified SSA: This type of SSA specifies only the segment name without any additional criteria. It directs IMS to retrieve the first occurrence of that segment type. For example - SEGMENT-NAME
  • Qualified SSA: This SSA includes the segment name along with specific conditions that the segment must satisfy. It allows for targeted retrieval of segment occurrences that meet the specified criteria. For example - SEGMENT-NAME (FIELD-NAME OPERATOR VALUE)

30. How can we access the data in IMS DB?

There are three main ways to access data in IMS DB:

  • Hierarchical Navigation (standard DL/I calls): Start from the root and navigate down to the required child segments.
  • Secondary Indexes: Use alternate paths to reach a segment (like using another field instead of the primary key).
  • Logical Relationships: Link data from different segments or databases logically (without following the physical hierarchy).