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Topic 7: File Organization

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Topic 7: File Organization Logical vs. Physical Organization of Data logical organization the abstract way that the computer program is able to access the data use of ... – PowerPoint PPT presentation

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Title: Topic 7: File Organization


1
Topic 7 File Organization
2
Logical vs. Physical Organization of Data
  • logical organization
  • the abstract way that the computer program is
    able to access the data
  • use of logical structures (e.g. linked lists)
  • physical organization
  • the actual physical structure of data in memory
  • i.e. what the sequence of bits look like in memory

3
Definitions
  • database
  • collection of related files
  • file
  • collection of related records
  • record
  • collection of related fields (e.g. Name, Age)
  • key field
  • uniquely identifies a record (e.g. UserID)

4
Basics (General Idea)
  • Records are stored at different places (different
    indices or locations)
  • To find a record, we need to know its location
  • We can search for the record
  • OR
  • Jump to its location directly (if location is
    known)
  • OR
  • A combination of jumping and searching

5
Sequential File Organization
  • Records in a file are stored sequentially (in
    order) by some key field
  • 2480 Bob
  • 2569 Alice
  • 3020 Paul
  • Originally designed to operate on magnetic tapes
  • How do we find a record?
  • What happens when we try to add a new record?
    (Its going to be bad)

6
Partially-Indexed Sequential Files
  • File index (address) index in a book
  • Partially index all the records
  • Key field has direct index to a section where
    record of interest is located
  • Sequential search for key field
  • Directly link to section of records
  • Sequential search for record of interest

7
Partially-Indexed Sequential Files
Key Record Address
A 1
B 6
C 11
D 16
Record
1
2
3
4
5
6
7
8
9
10
11
12
8
Fully Indexed Files
  • Every record has an index (address)
  • Sequentially search through key field for
    specific record address
  • Records may be accessed directly OR in sequential
    order by address

9
Fully Indexed Files
Key Record Address
a 4
b 7
c 5
d 3
e 12
m 9
n 10
p 2
s 11
t 6
z 1
10
Direct Access File Organization
  • Record address is derived/calculated with math
  • No need to search through an index
  • Example
  • Record Address UserID MOD 8 SSN MOD 3
  • Record Address UserID8 SSN3
  • This math operation is called key hashing or
    hashing

11
Fixed-length vs. Variable-length Records
  • Fixed-length
  • each record is a set size
  • can be used with direct access file organization
  • access based on math calculations, so size must
    be fixed in length
  • Variable-length
  • each record is a variable size
  • can be used with sequential file organization
  • access is all indexed, so size does not matter
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