Indexed sequenced access method in database

2.  ISAM (Indexed Sequential Access Method) is a
file management system developed at IBM that
allows records to be accessed either
sequentially (in the order they were entered) or
randomly (with an index).
 Each index defines a different ordering of the
records.
 An employee database may have several
indexes, based on the information being
sought.

3.  For example, a name index may order
employees alphabetically by last name, while a
department index may order employees by
their department.

4.  Two Types-
1) Static Hashing
2)Dynamic Hashing
- Extendable Hashing
- Linear Hashing

5. • In static hashing, when a search-key value is provided,
the hash function always computes the same address.
• The number of records are constant in static Hashing .

6.  Insertion − When a record is required to be
entered using static hash, the hash
function h computes the bucket address for search
key K, where the record will be stored.
 Bucket address = h(K)
 Search − When a record needs to be retrieved, the
same hash function can be used to retrieve the
address of the bucket where the data is stored.
 Delete − This is simply a search followed by a
deletion operation.

7.  Extendible hashing is designed to allow for
dynamic hash files, i.e. a hash file where the
number of records is not known in advance.
 Extendible hashing uses a directory to point
into the hash file.
 The directory points to the logical structure of
the hash file, while the blocks in the hash file
might be differently allocated.

8. • A large data file requiring a directory to span
several pages.
 Linear hashing allows for the expansion of the
hash table one slot at a time.
 The frequent single slot expansion can very
effectively control the length of the collision
chain.

9. – Static Hashing :When the number of records are constant
.
– Linear Hashing: When number of records varies, and hash
key values are uniformly distributed.

10. – Extendible Hashing : When data values lead to a skewed
distribution
– Linear Hashing : A large data file requiring a directory to
span several pages.

11.  A B+ tree is a balanced tree in which every
path from the root of the tree to a leaf is of the
same length, and each non-leaf node of the tree
has between [n/2] and [n] children, where n is
fixed for a particular tree.
 It contains index pages and data pages.
 The capacity of a leaf has to be 50% or more.
 For example: if n = 4, then the key for each
node is between 2 to 4. The index page will be 4
+ 1 = 5.

12. – Linear Hashing :When queries are equality search.
– B+ trees :When queries are range search.