There are two types of ordered indices: dense and sparse. A dense index contains an index record for every search key value with a pointer to the actual record, while a sparse index only creates index records for some records and sequential searching is needed to find the desired record. Dense indices require more space but allow faster access, making them preferable for secondary indices or when the index is small compared to memory.

1. 1
Dense and Sparse Indices
There are Two types of ordered indices:
Dense Index:
• An index record appears for every search key value in file.
• This record contains search key value and a pointer to the actual
record.
Sparse Index:
• Index records are created only for some of the records.
• We start at that record pointed to by the index record, and proceed
along the pointers in the file (that is, sequentially) until we find the
desired record.

2. Figures 1 and 2 show dense and sparse indices for the deposit file.
Figure 1: Dense index.
•Notice how we would find records for Perryridge branch using both methods.
Figure 2: Sparse index.
2

3. 3
Index Choice
• Dense index requires more space overhead and more
memory.
• Data can be accessed in a shorter time using Dense
Index.
• It is preferable to use a dense index when the file is
using a secondary index, or when the index file is
small compared to the size of the memory.