Topic 7: File Organization

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)

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 What happens when we try to add a new record?

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 1. Sequential search for key field 2. Directly link to section of records 3. Sequential search for record of interest

Partially-Indexed Sequential Files KeyRecord Address A1 B6 C11 D16 Record

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

Fully Indexed Files KeyRecord Address a4 b7 c5 d3 e12 m9 n10 p2 s11 t6 z1

Direct Access File Organization Record address is derived (calculated) from key field No need to search through an index Example: Record Address = UserID MOD 8 + SSN MOD 3 This math operation is called “key hashing” or “hashing”

Fixed-length vs. Variable-length Records Fixed-length  each record is a set size  can be used with direct 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

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