Slide 1

Introduction to Direct Access Files.

Slide 2

Sequential Files - Adding a Record Rec085 Rec300Rec150Rec005Rec090Rec045Rec100Rec001Rec325^Z Ordered Rec001Rec005Rec045Rec090Rec100Rec150Rec300Rec325^Z Unordered

Slide 3

Rec085 Rec300Rec150Rec005Rec090Rec045Rec100Rec001Rec325Rec085^Z New-Ordered Rec001Rec005Rec045Rec085Rec090Rec100Rec150Rec300Rec325^Z Extend-Unordered

Slide 4

Sequential Files - Deleting a Record Rec150 Rec300Rec150Rec005Rec090Rec045Rec100Rec001Rec325^Z Ordered Rec001Rec005Rec045Rec090Rec100Rec150Rec300Rec325^Z Unordered

Slide 5

Rec150 Rec300Rec005Rec090Rec045Rec100Rec001Rec325^Z New-Ordered Rec001Rec005Rec045Rec090Rec100Rec300Rec325^Z New-Unordered

Slide 6

Sequential Files - Amending a Record Rec045 Rec300Rec150Rec005Rec090Rec045Rec100Rec001Rec325^Z Ordered Rec001Rec005Rec045Rec090Rec100Rec150Rec300Rec325^Z Unordered

Slide 7

Rec045 Rec300Rec150Rec005Rec090Rec045Rec100Rec001Rec325^Z New-Ordered Rec001Rec005Rec045Rec090Rec100Rec150Rec300Rec325^Z New-Unordered

Slide 8

Relative Files - Organization Rec001freeRec003Rec004freefreeRec007Rec325Rec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number

Slide 9

Relative Files - Adding a Record Rec001freeRec003Rec004freefreeRec007Rec325Rec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec327

Slide 10

Relative Files - Adding a Record Rec001freeRec003Rec004freefreeRec007Rec325Rec326Rec327Rec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec327

Slide 11

Relative Files - Deleting a Record Rec001freeRec003Rec004freefreeRec007Rec325Rec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec325

Slide 12

Relative Files - Deleting a Record Rec001freeRec003Rec004freefreeRec007deleted/freeRec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec325

Slide 13

Relative Files - Amending a Record Rec001freeRec003Rec004freefreeRec007Rec325Rec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec007

Slide 14

Relative Files - Amending a Record Rec001freeRec003Rec004freefreeRec007Rec325Rec326freeRec328 1 2 3 4 5 6 7 325 326 327 328 Relative Record Number Rec007

Slide 15

Indexed Files - Organization H R Z L O R C F H T W Z Mi Nf Ni Nt Oi Ot Index Records Data Records

Slide 16

Indexed Files - Reading Record Ni H R Z L O R C F H T W Z Mi Nf Ni Nt Oi Ot Index Records Data Records

Slide 17

Indexed Files - Reading Record Ni H R Z L O R C F H T W Z Mi Nf Ni Nt Oi Ot Index Records Data Records

Slide 18

Indexed Files - Reading Record Ni H R Z L O R C F H T W Z Mi Nf Ni Nt Oi Ot Index Records Data Records

Slide 19

Sequential Files. Slow - when the hit rate is low. Complicated to change (insert, delete, amend) Fast - when the hit rate is high. Most storage efficient. Simple organization. Recovers space from deleted records. Disadvantages. Advantages.

Slide 20

Relative Files. Wasteful of storage if the file is only partially populated. Cannot recover space from deleted records. Only a single, numeric key allowed. Keys must map on to the range of the Relative Record numbers. Fastest Direct Access organization. Very little storage overhead. Can be read sequentially. Disadvantages. Advantages.

Slide 21

Indexed Files. Slowest Direct Access organization. Especially slow when adding or deleting records. Not very storage efficient. Must store the Index records, the alternate Index records, the data records and the alternate data records. Can use multiple, alphanumeric keys. Can have duplicate alternate keys. Can be read sequentially on any of its keys. Can partially recover space from deleted records. Disadvantages. Advantages.