SqlServer clustered index storage ( >1 columns )? - sql-server

Question

lets say i have a table like this :
a | b | c | d
______________
1 | 2 | 4 | 5
6 | 2 | 5 | 5
3 | 5 | 2 | 5
[a] column has clustered index
so the physical order which its stored is :
a | b | c | d
______________
1 | 2 | 4 | 5
3 | 5 | 2 | 5
6 | 2 | 5 | 5
now lets enhance the [a] index to be [a,c] ( still as clustered).
now , I can't udnerstand how it can be stored since [a] column is already sorted and [c] column cant be sorted ( because sorting of [a] hurts the sorting of [c])
so how does sqlServer will store it ?
2'nd question : do I need to open another index for [c] ?

Answer 1

I think you're missing something obvious. Consider what you would expect from the query
select * from myTable
order by [a], [c]
Your clustered index on columns [a,c] will give a physical layout with the same order.

Answer 2

Composite indexes produce lexicographical order: the records are additionally ordered on c when values of a are considered "equal".
a c
1 2
2 3 -- Within this block, records are sorted on [c]
2 5 --
2 7 --
3 7
4 1
5 6 -- Within this block, records are sorted on [c]
5 8 --
This is how dictionaries sort.
You need an additional index on c if you want to speed up queries not involving a:
SELECT *
FROM mytable
WHERE c = #some_value