What is columnstore index and how is different from clustered and non-clustered

Question

Hi I am confused about columnstore index, what is column store index and how it is different from clustered and non-clustered index..

Answer 1

Assume you have a table like below with col1 as primary key

col1  col2  col3
1      2     3
4      5     6

Normal index will be stored like below,assuming a page can hold only one row

   row1  1   2  3--page1-- all columns reside in one page
   row2  4   5  6--page2
   

so when you want to read some thing like sum(col3),SQLServer will need to read page1 and page 2 ,that's a cost of two pages..

Now with column store indexes,The same table will be stored like below

page1  page2   page3
1       2       3
4       5       6

Now if you want to do a sum of col3,it just has to read one page(page3)

Benefit of using column store indexes is, you may touch only necessary pages from Disk .Memory is also efficiently used ,since you will not be storing/reading unwanted data