Describe the storage structure of indexed sequential files and their access method.
Ans: Index offers a lookup capability to rapidly reach the vicinity of the desired record.
- consists of key field and a pointer to the main file
- Indexed is searched to find out highest key value that is equal to or precedes the desired key value
- Search carries on in the main file at the location pointed out by the pointer.
If an index consists of 1000 entries, it will take on average 500 accesses to find out the key, followed by 500 accesses in the main file. Now on average it is 1000 accesses.
a) New records are added to an overflow file
b) Record in main file which precedes it is updated to include a pointer to the new record
c) The overflow is merged along with the main file during a batch update
d) Multiple indexes for similar key field can be set up to increase efficiency.
ISAM that is Indexed Sequential Access Method is a file management system developed at IBM that permits records to be accessed either sequentially (in the order they were entered) or randomly (with an index). Every index defines a different ordering of the records. An employee database may have various indexes, based on the information being sought. For instance, a name index may order employees alphabetically by last name, when a department index may order employees by their department. A key is fixed in each index. For an alphabetical index of employee names, the last name field would be the key.