Indexed sequential access method is the organization of file in an advanced sequential manner. A primary key is used in this method to store records in the file. Also, it generates an index value, and mapping is done with the record.
This index stores the address of the file.
If we use the index value for retrieving a data, first fetching of address takes place, and then the record gets retrieved.
Advantages of ISAM
- The searching of the record is easy as there is an address for each data block.
- Range and partial retrieval of records are supported by this method. We can obtain data for all the values as the basis of index is primary values. Similarly, we can also search for any partial value. For example, we can search employee names starting with PA.
Disadvantages of ISAM
- Index value needs extra space for its storage.
- File reconstruction takes place every time we enter a new data for maintaining the sequence.
- We need to release the space once a record gets deleted. If not released, the database performance gets slow.