sql - How do composite indexes work?

Question

I've created composite indexes (indices for you mathematical folk) on tables before with an assumption of how they worked. I was just curious if my assumption is correct or not.

I assume that when you list the order of columns for the index, you are also specifying how the indexes will be grouped. For instance, if you have columns a, b, and c, and you specify the index in that same order a ASC, b ASC, and c ASC then the resultant index will essentially be many indexes for each "group" in a.

Is this correct? If not, what will the resultant index actually look like?

Answer 1

Composite indexes work just like regular indexes, except they have multi-values keys.

If you define an index on the fields (a,b,c) , the records are sorted first on a, then b, then c.

Example:

| A | B | C |
-------------
| 1 | 2 | 3 |
| 1 | 4 | 2 |
| 1 | 4 | 4 |
| 2 | 3 | 5 |
| 2 | 4 | 4 |
| 2 | 4 | 5 |