Choosing an organisation and a route

Overview

Cost of maintaining an index, trade-off in choosing the size of a hashed address space and balanced conclusion about file organisation.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why is there no single best file organisation?

   Model answer

   The right choice depends on the hit rate, whether records are needed in key order, and how often the file changes.

2. 2

   Which organisation suits a file processed in full each run and never accessed singly?

   Model answer

   Sequential, because it needs no index and reads at the medium's fastest rate.

3. 3

   Which organisation suits a file where single records are looked up at random and never listed in order?

   Model answer

   Hashed direct access, because it reaches any record in one access.

4. 4

   Which organisation suits a file needing both a monthly ordered report and instant single lookups?

   Model answer

   Indexed sequential, which supports both at the cost of maintaining the index.

5. 5

   What is the cost of maintaining an index?

   Model answer

   Every insertion and deletion must update the index as well as the file, so writes are slower.

6. 6

   Why does a heavily updated hashed file degrade faster than a heavily updated indexed one?

   Model answer

   Overflow is not tidied as it accumulates, whereas an index can be adjusted as records are added.

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