In file organization methods, what does Indexed-Sequential access (ISAM-style) mean in practice: how are records stored and how are they located?

Introduction / Context:
Classic files can be organized sequentially, directly (hashed), or indexed-sequential. Indexed-Sequential Access Method (ISAM) combines the strengths of sequential storage (efficient batch processing) with indexing for faster keyed retrieval, a foundational concept behind modern indexed tables.

Given Data / Assumptions:

• Records are maintained in a logical key order.
• An index exists to accelerate searches, inserts, and updates.
• Overflow areas or level-2 indexes may handle growth.

Concept / Approach:

In indexed-sequential organization, records are stored sequentially (usually ordered by a primary key). An index maps key ranges or specific keys to physical locations, enabling near-random access for lookups while still supporting efficient sequential scans for reporting and batch jobs. This hybrid approach predates and informs B-tree indexes used in relational databases.

Step-by-Step Solution:

Verification / Alternative check:

ISAM/VSAM documentation describes an index plus sequential data set, often with overflow handling and periodic reorganization.

Why Other Options Are Wrong:

Random placement contradicts sequential order.

“Index for each key type” is not required; typically there is a primary index, with optional secondary indexes.

“Contiguous blocks according to a key” is overly restrictive and not the defining characteristic.

Common Pitfalls:

Allowing unchecked overflow growth which degrades performance; plan reorganizations to maintain locality and index efficiency.

Final Answer:

Stores records sequentially but uses an index to locate records