|
View previous topic :: View next topic
|
| Author |
Message |
barthwalsamarth
New User
.jpg)
Joined: 03 Aug 2006
Posts: 9
|
|
|
|
Hi I am new to mainframes.
I have a doubt that why we need indexes.
Primary keys are used to identify unique rows but I read that index can be same for more than one row, then what was the purpose for which we created the index.
Thanks in advance!!! |
|
| Back to top |
|
 |
Craq Giegerich
Senior Member
Joined: 19 May 2007
Posts: 1512
Location: Virginia, USA
|
|
|
|
| Speed! Indexes are maintained in sequence and using an index avoids scanning the entire table looking for a value. |
|
| Back to top |
|
|
dick scherrer
Moderator Emeritus
Joined: 23 Nov 2006
Posts: 19244
Location: Inside the Matrix
|
|
|
|
Hello,
For this
| Quote: |
| I read that index can be same for more than one row |
an example might be in a "personnel" system.
Each person-id is unique, but let's say each department may have between 0 and many people assigned to it. If dept_no was a secondary key (index) a query could directly obtain all of the people assigned to that department and there would be no extra overhead traversing the entire table to return only those people assigned to the particular department(s) used in the WHERE. It could very quickly also determine that there were no people assigned to "that" department. |
|
| Back to top |
|
|
|
|
