Slide 13.14: Improving the secondary index structures

Improving the Secondary Index Structures

Secondary indexes lead to two difficulties:

All index files have to be rearranged every time a new record is added to the data file.
If there are duplicate secondary keys, the secondary key field is repeated for each entry—space is wasted.

A First Attempt at a Solution

Change the secondary index structure so it associates an array of references with each secondary key.

Advantages: It helps avoid the need to rearrange the secondary index file. For example, if we want to add the following record:
```
   ANG 36193 Piano Concerto 3 and 5 Prokofiev Francois
```
the corresponding secondary index record is simply modified to:
```
   PROKOFIEV             ANG36193  LON2312
```
Disadvantages:
- The number of references associated with each secondary key may be restricted.
- By extending the fixed length of each of the secondary index records to hold more reference fields, we might lose more space due to internal fragmentation.