February 02, 2020
Amitraj
Indexed sequential access method (ISAM)
-> This is an advanced sequential file organization method. Here, records are stored in order of primary key in the file. Using the primary key, the records are sorted.
-> For each primary key, an index value is generated and mapped with the record. This index is nothing but the address of record in the file.
-> If any record has to be retrieved based on its index value, then the address of the data block is fetched and the record is retrieved from the memory.
Pros of ISAM -
1. Since each record has its data block address, searching for a record in larger database is easy and quick.
2. This method supports range retrieval and partial retrieval of records. Since the index is based on the primary key values, we can retrieve the data for the given range of value. In the same way, the partial value can also be easily searched, i.e., the student name starting with 'JA' can be easily searched.
Cons of ISAM -
1. This method requires extra space in the disk to store the index value.
2. When the new records are inserted, then these files have to be reconstructed to maintain the sequence.
3. When the record is deleted, then the space used by it needs to be released. Otherwise, the performance of the database will slow down.
-> This is an advanced sequential file organization method. Here, records are stored in order of primary key in the file. Using the primary key, the records are sorted.
-> For each primary key, an index value is generated and mapped with the record. This index is nothing but the address of record in the file.
-> If any record has to be retrieved based on its index value, then the address of the data block is fetched and the record is retrieved from the memory.
Pros of ISAM -
1. Since each record has its data block address, searching for a record in larger database is easy and quick.
2. This method supports range retrieval and partial retrieval of records. Since the index is based on the primary key values, we can retrieve the data for the given range of value. In the same way, the partial value can also be easily searched, i.e., the student name starting with 'JA' can be easily searched.
Cons of ISAM -
1. This method requires extra space in the disk to store the index value.
2. When the new records are inserted, then these files have to be reconstructed to maintain the sequence.
3. When the record is deleted, then the space used by it needs to be released. Otherwise, the performance of the database will slow down.
