Database system

DATABASE SYSTEM
Entity relationship model, Components of ER model, Mapping E-R
model to Relational schema, Network and Object Oriented Data
models, Storage Strategies: Detailed Storage Architecture, Storing
Data, Magnetic Disk, RAID, Other Disks, Magnetic Tape, Storage
Access, File & Record Organization, File Organizations & Indexes,
Order Indices, B+ Tree Index Files, Hashing Data Dictionary
By Hitesh Mohapatra

Network and Object Oriented Data models
“A data model is a way of finding the tools for both business and IT professionals, which
uses a set of symbols and text to precisely explain a subset of real information to improve
communication within the organization and thereby lead to a more flexible and stable
application environment”.
A data model is an idea which describes how the data can be represented and accessed
from software system after its complete implementation.
• It is a simple abstraction of complex real world data gathering environment.
• It defines data elements and relationships among various data elements for a specified
system.
• The main purpose of data model is to give an idea that how final system or software will
look like after development is completed.

Types of Data Model
Following are the types of Data Model,
1. Hierarchical Model
2. Relational Model
3. Network Database Model
4. Entity Relationship Model
5. Object Model

Hierarchical Model
• Hierarchical model was developed by IBM and North
American Rockwell known as Information Management
System.
• It represents the data in a hierarchical tree structure.
• This model is the first DBMS model.
• In this model, the data is sorted hierarchically.
• It uses pointer to navigate between the stored data.

Relational Model
• Relational model is based on first-order predicate logic.
• This model was first proposed by E. F. Codd.
• It represents data as relations or tables.
• Relational database simplifies the database structure by making
use of tables and columns.

Network Database Model
• Network Database Model is same like Hierarchical Model, but the only
difference is that it allows a record to have more than one parent.
• In this model, there is no need of parent to child association like the hierarchical
model.
• It replaces the hierarchical tree with a graph.
• It represents the data as record types and one-to-many relationship.
• This model is easy to design and understand.

Entity Relationship Model
•Entity Relationship Model is a high-level data model.
•It was developed by Chen in 1976.
•This model is useful in developing a conceptual design for the database.
•It is very simple and easy to design logical view of data.
•The developer can easily understand the system by looking at an ER model
constructed.
• In this diagram Rectangle represents the entities. Eg. Doctor
and Patient.
• Ellipse represents the attributes. Eg. DocId, Dname, PId, Pname.
Attribute describes each entity becomes a major part of the
data stored in the database.
• Diamond represents the relationship in ER diagrams. Eg.
Doctor diagnoses the Patient.

Object Model
• Object model stores the data in the form of objects, classes and inheritance.
• This model handles more complex applications, such as Geographic
Information System (GIS), scientific experiments, engineering design and
manufacturing.
• It is used in File Management System.
• It represents real world objects, attributes and behaviours.
• It provides a clear modular structure.
• It is easy to maintain and modify the existing code.

Ex.
Imagine we have to design database for college. What is the real world entities involved with
college? They are college, Students, Lecturer, Course, Subject, Marks etc. Once all the entities
are listed, we find out the relationship between them and try to map all of them. Also we list
what are the attributes related to each entity like student id, name, lecturer name, course
that he is teaching, different subjects, pass mark, grade levels etc. Here we are not bothered
about what data value is stored, what is the size of each data etc. We know only entities
involved, their attributes and mapping at this stage.

Object Oriented Data Models
• This data model is another method of representing real world objects.
• It considers each object in the world as objects and isolates it from each other.
• It groups its related functionalities together and allows inheriting its functionality to other
related sub-groups.

Ex.
Let us consider an Employee database to understand this model better. In this database we have
different types of employees – Engineer, Accountant, Manager, Clark. But all these employees belong
to Person group. Person can have different attributes like name, address, age and phone. What do we
do if we want to get a person’s address and phone number? We write two separate procedure
sp_getAddress and sp_getPhone.

Ex..Cont..
What about all the employees above? They too have all the attributes what a person has. In addition, they
have their EMPLOYEE_ID, EMPLOYEE_TYPE and DEPARTMENT_ID attributes to identify them in the
organization and their department. We have to retrieve their department details, and hence we
sp_getDeptDetails procedure. Currently, say we need to have only these attributes and functionality. Since
all employees inherit the attributes and functionalities of Person, we can re-use those features in Employee.
But do we do that? We group the features of person together into class. Hence a class has all the attributes
and functionalities. For example, we would create a person class and it will have name, address, age and
phone as its attribute, and sp_getAddress and sp_getPhone as procedures in it. The values for these
attributes at any instance of time are object. i.e. ; {John, Troy, 25, 2453545 : sp_getAddress (John),
sp_getPhone (John)} forms on person object. {Mathew, Fraser Town, 28, 5645677: sp_getAddress (Mathew),
sp_getPhone (Mathew} forms another person object.
Now, we will create another class called Employee which will inherit all the functionalities of Person class. In
addition it will have attributes EMPLOYEE_ID, EMPLOYEE_TYPE and DEPARTMENT_ID, and sp_getDeptDetails
procedure. Different objects of Employee class are Engineer, Accountant, Manager and Clerk.

Ex.Cont..
It would be a black box to any other
classes. This feature of this model is
called encapsulation. It binds the
features in one class and hides it
from other classes. It is only visible to
its objects and any inherited classes.

Advantages of OOM
• Because of its inheritance property, we can re-use the attributes and functionalities. It reduces the
cost of maintaining the same data multiple times. Also, these information are encapsulated and, there
is no fear being misused by other objects. If we need any new feature we can easily add new class
inherited from parent class and adds new features. Hence it reduces the overhead and maintenance
costs.
• Because of the above feature, it becomes more flexible in the case of any changes.
• Codes are re-used because of inheritance.
• Since each class binds its attributes and its functionality, it is same as representing the real world
object.
• We can see each object as a real entity. Hence it is more understandable.

Disadvantages of OOM
• It is not widely developed and complete to use it in the database systems. Hence it is not accepted by
the users.
• It is an approach for solving the requirement. It is not a technology. Hence it fails to put it in the
database management systems.

Storage Strategies
1. Detailed Storage Architecture
2. Storing Data
3. Magnetic Disk
4. RAID
5. Other Disks, Magnetic Tape, Storage Access
6. File & Record Organization, File Organizations & Indexes

DBMS - Storage System
• Databases are stored in file formats, which contain records. At physical level, the actual data is stored in
electromagnetic format on some device. These storage devices can be broadly categorized into three
types −

Primary Storage
• The memory storage that is directly accessible to the CPU comes under this category.
• CPU's internal memory (registers), fast memory (cache), and main memory (RAM) are directly accessible
to the CPU, as they are all placed on the motherboard or CPU chipset.
• This storage is typically very small, ultra-fast, and volatile. Primary storage requires continuous power
supply in order to maintain its state.
• In case of a power failure, all its data is lost.

Secondary Storage
• Secondary storage devices are used to store data for future use or as backup.
• Secondary storage includes memory devices that are not a part of the CPU chipset or motherboard,
for example, magnetic disks, optical disks (DVD, CD, etc.), hard disks, flash drives, and magnetic tapes.

Tertiary Storage
• Tertiary storage is used to store huge volumes of data.
• Since such storage devices are external to the computer system, they are the slowest in speed.
• These storage devices are mostly used to take the back up of an entire system.
• Optical disks and magnetic tapes are widely used as tertiary storage.

Memory Hierarchy
A computer system has a well-defined hierarchy of memory. A CPU has direct access to it main
memory as well as its inbuilt registers. The access time of the main memory is obviously less
than the CPU speed. To minimize this speed mismatch, cache memory is introduced. Cache
memory provides the fastest access time and it contains data that is most frequently accessed
by the CPU.
The memory with the fastest access is the costliest one. Larger storage devices offer slow speed
and they are less expensive, however they can store huge volumes of data as compared to CPU
registers or cache memory.

Magnetic Disks
Hard disk drives are the most common secondary storage
devices in present computer systems. These are called
magnetic disks because they use the concept of
magnetization to store information. Hard disks consist of
metal disks coated with magnetizable material. These
disks are placed vertically on a spindle. A read/write head
moves in between the disks and is used to magnetize or
de-magnetize the spot under it. A magnetized spot can
be recognized as 0 (zero) or 1 (one).
Hard disks are formatted in a well-defined order to store
data efficiently. A hard disk plate has many concentric
circles on it, called tracks. Every track is further divided
into sectors. A sector on a hard disk typically stores 512
bytes of data.

Redundant Array of Independent Disks
RAID or Redundant Array of Independent Disks, is a technology to connect multiple secondary
storage devices and use them as a single storage media.
RAID consists of an array of disks in which multiple disks are connected together to achieve
different goals. RAID levels define the use of disk arrays.

RAID 0
In this level, a striped array of disks is implemented. The data is broken down into blocks and
the blocks are distributed among disks. Each disk receives a block of data to write/read in
parallel. It enhances the speed and performance of the storage device. There is no parity and
backup in Level 0.
RAID 0

RAID 1
RAID 1 uses mirroring techniques. When data is sent to a RAID controller, it sends a copy of data to all the
disks in the array. RAID level 1 is also called mirroring and provides 100% redundancy in case of a failure.

RAID 2
RAID 2 records Error Correction Code using Hamming distance for its data, striped on different
disks. Like level 0, each data bit in a word is recorded on a separate disk and ECC codes of the
data words are stored on a different set disks. Due to its complex structure and high cost, RAID
2 is not commercially available.

RAID 3
RAID 3 stripes the data onto multiple disks. The parity bit generated for data word is stored on a
different disk. This technique makes it to overcome single disk failures.

RAID 4
In this level, an entire block of data is written onto data disks and then the parity is generated
and stored on a different disk. Note that level 3 uses byte-level striping, whereas level 4 uses
block-level striping. Both level 3 and level 4 require at least three disks to implement RAID.

RAID 5
RAID 5 writes whole data blocks onto different disks, but the parity bits generated for data block
stripe are distributed among all the data disks rather than storing them on a different dedicated
disk.

RAID 6
RAID 6 is an extension of level 5. In this level, two independent parities are generated and stored
in distributed fashion among multiple disks. Two parities provide additional fault tolerance. This
level requires at least four disk drives to implement RAID.

DBMS - File Structure
Relative data and information is stored collectively in file formats. A file is a sequence of records
stored in binary format. A disk drive is formatted into several blocks that can store records. File
records are mapped onto those disk blocks.

File Organization
File Organization defines how file records are mapped onto disk blocks. We have four types of
File Organization to organize file records −

Heap File Organization
When a file is created using Heap File Organization, the Operating System allocates memory
area to that file without any further accounting details. File records can be placed anywhere in
that memory area. It is the responsibility of the software to manage the records. Heap File does
not support any ordering, sequencing, or indexing on its own.

Sequential File Organization
Every file record contains a data field (attribute) to uniquely identify that record. In sequential
file organization, records are placed in the file in some sequential order based on the unique key
field or search key. Practically, it is not possible to store all the records sequentially in physical
form.

Hash File Organization
Hash File Organization uses Hash function computation on some fields of the records. The
output of the hash function determines the location of disk block where the records are to be
placed.

Clustered File Organization
Clustered file organization is not considered good for large databases. In this mechanism,
related records from one or more relations are kept in the same disk block, that is, the ordering
of records is not based on primary key or search key.

File Operations
Operations on database files can be broadly classified into two categories −
• Update Operations
• Retrieval Operations
Update operations change the data values by insertion, deletion, or update. Retrieval operations,
on the other hand, do not alter the data but retrieve them after optional conditional filtering. In
both types of operations, selection plays a significant role.

Other Operations
Other than creation and deletion of a file, there could be several operations, which can be done
on files.
Open − A file can be opened in one of the two modes, read mode or write mode. In read mode,
the operating system does not allow anyone to alter data. In other words, data is read only. Files
opened in read mode can be shared among several entities. Write mode allows data
modification. Files opened in write mode can be read but cannot be shared.
Locate − Every file has a file pointer, which tells the current position where the data is to be
read or written. This pointer can be adjusted accordingly. Using find (seek) operation, it can be
moved forward or backward.
Read − By default, when files are opened in read mode, the file pointer points to the beginning
of the file. There are options where the user can tell the operating system where to locate the
file pointer at the time of opening a file. The very next data to the file pointer is read.

Other Operations
Write − User can select to open a file in write mode, which enables them to edit its contents. It
can be deletion, insertion, or modification. The file pointer can be located at the time of opening
or can be dynamically changed if the operating system allows to do so.
Close − This is the most important operation from the operating system’s point of view. When a
request to close a file is generated, the operating system
• removes all the locks (if in shared mode),
• saves the data (if altered) to the secondary storage media, and
• releases all the buffers and file handlers associated with the file.
The organization of data inside a file plays a major role here. The process to locate the file
pointer to a desired record inside a file various based on whether the records are arranged
sequentially or clustered.

DBMS - Indexing
We know that data is stored in the form of records. Every record has a key field, which helps it to
be recognized uniquely.
Indexing is a data structure technique to efficiently retrieve records from the database files
based on some attributes on which the indexing has been done. Indexing in database systems is
similar to what we see in books.

Indexing
Indexing is defined based on its indexing attributes. Indexing can be of the following types −
Primary Index − Primary index is defined on an ordered data file. The data file is ordered on a
key field. The key field is generally the primary key of the relation.
Secondary Index − Secondary index may be generated from a field which is a candidate key
and has a unique value in every record, or a non-key with duplicate values.
Clustering Index − Clustering index is defined on an ordered data file. The data file is ordered
on a non-key field.
Ordered Indexing is of two types −
Dense Index
Sparse Index

Dense Index
In dense index, there is an index record for every search key value in the database. This makes
searching faster but requires more space to store index records itself. Index records contain
search key value and a pointer to the actual record on the disk.

Sparse Index
In sparse index, index records are not created for every search key. An index record here
contains a search key and an actual pointer to the data on the disk. To search a record, we first
proceed by index record and reach at the actual location of the data. If the data we are looking
for is not where we directly reach by following the index, then the system starts sequential
search until the desired data is found.

Multilevel Index
Index records comprise search-key values
and data pointers. Multilevel index is stored
on the disk along with the actual database
files. As the size of the database grows, so
does the size of the indices. There is an
immense need to keep the index records in
the main memory so as to speed up the
search operations. If single-level index is
used, then a large size index cannot be
kept in memory which leads to multiple
disk accesses.
Multi-level Index helps in breaking down the index into several
smaller indices in order to make the outermost level so small
that it can be saved in a single disk block, which can easily be
accommodated anywhere in the main memory.

B+ Tree Index Files
A B+ tree is a balanced binary search tree that follows a multi-level index format. The leaf nodes
of a B+ tree denote actual data pointers. B+ tree ensures that all leaf nodes remain at the same
height, thus balanced. Additionally, the leaf nodes are linked using a link list; therefore, a B+ tree
can support random access as well as sequential access.

Structure of B+ Tree
Every leaf node is at equal distance from the root node. A B+ tree is of the order n where n is fixed for every
B+ tree.
Internal nodes −
Internal (non-leaf) nodes contain at least ⌈n/2⌉
pointers, except the root node. At most, an
internal node can contain n pointers.
Leaf nodes −
Leaf nodes contain at least ⌈n/2⌉ record pointers and
⌈n/2⌉ key values. At most, a leaf node can contain n
record pointers and n key values. Every leaf node
contains one block pointer P to point to next leaf
node and forms a linked list.

B+ Tree Insertion
•If a leaf node overflows −
•Split node into two parts.
•Partition at i = ⌊(m+1)/2⌋.
•First i entries are stored in one node.
•Rest of the entries (i+1 onwards) are moved to a new node.
•ith key is duplicated at the parent of the leaf.
•If a non-leaf node overflows −
•Split node into two parts.
•Partition the node at i = ⌈(m+1)/2⌉.
•Entries up to i are kept in one node.
•Rest of the entries are moved to a new node.
•B+ trees are filled from bottom and each entry is done at the leaf node.

B+ Tree Deletion
•B+ tree entries are deleted at the leaf nodes.
•The target entry is searched and deleted.
•If it is an internal node, delete and replace with the entry from the left position.
•After deletion, underflow is tested,
•If underflow occurs, distribute the entries from the nodes left to it.
•If distribution is not possible from left, then
•Distribute from the nodes right to it.
•If distribution is not possible from left or from right, then
•Merge the node with left and right to it.

DBMS - Hashing
For a huge database structure, it can be almost next to impossible to search all the index values
through all its level and then reach the destination data block to retrieve the desired data.
Hashing is an effective technique to calculate the direct location of a data record on the disk
without using index structure.
Hashing uses hash functions with search keys as parameters to generate the address of a data
record.

Hash Organization
Bucket − A hash file stores data in bucket format. Bucket is considered a unit of storage. A
bucket typically stores one complete disk block, which in turn can store one or more records.
Hash Function − A hash function, h, is a mapping function that maps all the set of search-keys
K to the address where actual records are placed. It is a function from search keys to bucket
addresses.

Static Hashing
In static hashing, when a search-key value is provided, the hash function always computes the same
address. For example, if mod-4 hash function is used, then it shall generate only 5 values. The output
address shall always be same for that function. The number of buckets provided remains unchanged at all
times.

Operations
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.

Bucket Overflow
The condition of bucket-overflow is known as collision. This is a fatal state for any static hash
function. In this case, overflow chaining can be used.
Overflow Chaining − When buckets are full, a new bucket is allocated for the same hash result and is
linked after the previous one. This mechanism is called Closed Hashing.
Linear Probing − When a hash function generates an address at which data is already stored, the next free
bucket is allocated to it. This mechanism is called Open Hashing.

Dynamic Hashing
The problem with static hashing is
that it does not expand or shrink
dynamically as the size of the
database grows or shrinks.
Dynamic hashing provides a
mechanism in which data buckets
are added and removed
dynamically and on-demand.
Dynamic hashing is also known
as extended hashing.
Hash function, in dynamic
hashing, is made to produce a
large number of values and only a
few are used initially.

Organization
The prefix of an entire hash value is taken as a hash index. Only a portion of the hash value is used for
computing bucket addresses. Every hash index has a depth value to signify how many bits are used for
computing a hash function. These bits can address 2n buckets. When all these bits are consumed − that is,
when all the buckets are full − then the depth value is increased linearly and twice the buckets are
allocated.

Operation
•Querying − Look at the depth value of the hash index and use those bits to compute the
bucket address.
•Update − Perform a query as above and update the data.
•Deletion − Perform a query to locate the desired data and delete the same.
•Insertion − Compute the address of the bucket
• If the bucket is already full.
• Add more buckets.
• Add additional bits to the hash value.
• Re-compute the hash function.
• Else
• Add data to the bucket,
• If all the buckets are full, perform the remedies of static hashing.
Hashing is not favorable when the data is organized in some ordering and the queries require a
range of data. When data is discrete and random, hash performs the best.
Hashing algorithms have high complexity than indexing. All hash operations are done in
constant time.

For Any Query
hiteshmohapatra@yahoo.com

Database system

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