Data - Organisation | Data processing

Relative file

Relative Access

• Features:
  • Actual location of a record in the file is determined by the value of specified key field(s).
• Examples:
  • The key value IS the location.

Random Access

• Features
  • Access is direct.
  • The same key value always converts to the same location.
  • Records all have same length
  • Access is very fast. To compute the record position in the file only needs a computation using a formula such as;
    (file_start_position + (record_number - 1) * record_length)
  • Records have no order
  • All (blank) records must be created when the file is created, and will contain many holes.
  • Note: Contention may occur - where two records hash to the same location.(Handled using a bucket for overflowing records. If the bucket becomes too large the hashing algorithm needs to be revised).
• Examples:
  • Divide key by prime number
  • Extract selected characters from key
  • With a numeric key, divide the key by the size of the file and use the remainder (+1) to get the location.