Topic 7: File Organization

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Topic 7 File Organization
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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

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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)

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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

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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)

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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

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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
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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

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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
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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

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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