Primary index
A primary index is a file that has a sorted series of records having two columns: a block address for that key field in the data file; and the ordering key field. The ordering key field for this primary index can be the primary key of the data file. Primary index have one index entry for every value of the ordering key field. An entry in primary index file has the index value of the pointer to that data block and a first record of the data block.
Let us talk about primary index with the help of an instance. Let us suppose a student database as (Supposing that one block keeps only four student records.):
|
Enrolment Progra- Number Name City mme
|
|
BLOCK 1
|
2109348
|
ANU VERMA
|
CHENNAI
|
CIC
|
|
2109349
|
ABHISHEK KUMAR
|
CALCUTTA
|
MCA
|
|
2109351
|
VIMAL KISHOR
|
KOCHI
|
BCA
|
|
2109352
|
RANJEETA JULIE
|
KOCHI
|
CIC
|
|
BLOCK 2
|
2109353
|
MISS RAJIYA BANU
|
VARANASI
|
MBA
|
|
2238389
|
JITENDAR KASWAN
|
NEW DELHI
|
MBA
|
|
2238390
|
RITURAJ BHATI
|
VARANASI
|
MCA
|
|
2238411
|
AMIT KUMAR JAIN
|
NEW DELHI
|
BCA
|
|
BLOCK 3
|
2238412
|
PAWAN TIWARI
|
AJMER
|
MCA
|
|
2238414
|
SUPRIYA SWAMI
|
NEW DELHI
|
MCA
|
|
2238422
|
KAMLESH KUMAR
|
MUMBAI
|
BSC
|
|
2258014
|
DAVEN SINGHAL
|
MUMBAI
|
BCA
|
|
BLOCK 4
|
2258015
|
S SRIVASTAVA
|
MUMBAI
|
BCA
|
|
2258017
|
SHWETA SINGH
|
NEW DELHI
|
BSC
|
|
2258018
|
ASHISH TIWARI
|
MUMBAI
|
MCA
|
|
2258019
|
SEEMA RANI
|
LUCKNOW
|
MBA
|
|
...
|
...
|
...
|
...
|
...
|
|
BLOCK r
|
2258616
|
NIDHI
|
AJMER
|
BCA
|
|
2258617
|
JAGMEET SINGH
|
LUCKNOW
|
MCA
|
|
2258618
|
PRADEEP KUMAR
|
NEW DELHI
|
BSC
|
|
2318935
|
RAMADHAR
|
FARIDABAD
|
MBA
|
|
...
|
...
|
...
|
...
|
...
|
|
BLOCK N-1
|
2401407
|
BRIJMISHRA
|
BAREILLY
|
CIC
|
|
2401408
|
AMIT KUMAR
|
BAREILLY
|
BSC
|
|
2401409
|
MD. IMRAN SAIFI
|
AURANGABAD
|
BCA
|
|
2401623
|
ARUN KUMAR
|
NEW DELHI
|
MCA
|
|
BLOCK N
|
2401666
|
ABHISHEK RAJPUT
|
MUMBAI
|
MCA
|
|
2409216
|
TANNUJ SETHI
|
LUCKNOW
|
MBA
|
|
2409217
|
SANTOSH KUMAR
|
ALMORA
|
BCA
|
|
2409422
|
SAKSHI GINOTRA
|
MUMBAI
|
BSC
|
Figure: A Student file stored in the order of student enrolment numbers