1. Database Management System
Lecture - 36
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2. Direct Access (Hashing)
Provides rapid, non-sequential, direct access to records.
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Hashing
A key record field is used to calculate the record address by subjecting it to some calculation; a process called hashing.
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Hashing
For numeric ascending order a sequential key record fields this might involve simply using relative address indexes from a base storage address to access records.
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Hashing
Most of the time, key field does not have the values in sequence that can directly be used as relative record number
Has to be transformed
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Hashing Algorithms
There are many
Two very well known are
Prime division/remainder method
Folding method
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Example
Suppose National ID Card number is the key for employees’ record; format is
New ID Card Numbers (hope you have them)
We take the middle part and transform them into a relative record number
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Hashing Example
Now the middle part consists of 7 digits can hold numbers up to one million, whereas our organization has maximum of 800 employees
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Hashing Example
Dividing the middle part with 1000 and using remainder as the hash number
999
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Folding Hash Algo
Key value:
Hashing method: Folding
Split number into 3–digits parts, except the middle one, like
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Folding Hashing Algo
Add these three components, we get 1150, take three least significant digits, so the key becomes 150
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Hashing Algo
Smart Algorithm is the one that generates the key values uniformly
COLLISIONS cannot be avoided altogether
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Key Collision
Like, applying previous algorithm on gives the value 1150
Considering the three least significant numbers the we get 150, which is collision
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Collision Handling
Records are arranged in buckets; collection of records
Keys are generated for buckets
Leave spare record places in buckets
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Collision Handling
If we expect 800 records, and with the 4 records per bucket we need 200 buckets, but rather than generating hash key up to 200, we fix the range as 250, this leaves extra space in each bucket
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Collision Handling
Find the place for collided record in the next buckets
OR leave a separate overflow area for collided records
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17. Direct Access (Bucket overflow)
An overflow area is set aside to deal with the bucket overflow problem
A synonym pointer at the end of the bucket area points to the first record in its corresponding overflow area.
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18. Direct Access (Bucket overflow)
Each record in the overflow area contains a next synonym pointer to a possible next record in that bucket overflow area.
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19. Direct Access (Bucket overflow)
Prime Area
Bucket 0
| 2500
5300
2200
3400
3800
4500|
Bucket 1
| 8901
7901
3201
5701 | …
…||
Bucket 2
|3902
4502
2202
Overflow Area
| | 5500
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20. Summary of Data Storage Concepts
Different types of storage devices are available for storing data
Different file organizations are available to store data.
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Indexes and Views
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Introduction
Sometimes, we want to retrieve records by specifying the values in one or more fields, e.g.,
Find all students in the “CS” department
Find all students with a gpa > 3.0
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Introduction
An index on a table is a disk-based data structure (stored as file) that speeds up selections on the search key fields for the index.
A schema object
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24. Database Management System
Lecture - 36
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