Question

Let’s say I have the following table:

Orders
======
OrderID
CustomerID
StatusID
DateCreated

And I have the following queries:

select CustomerID from Orders where OrderID = 100

select OrderID from Orders where CustomerID = 20

select OrderID, StatusID from Orders where CustomerID = 100 and OrderID = 1000

If I make the following index:

create nonclustered index Example
  On dbo.Orders(OrderID,CustomerID)
  Include(StatusID)

Does that take care of optimization of all 3 queries with one index? In other words, do composite indexes improve queries that use one of the items within the composite? Or should individual indexes be created just on those columns as well (ie OrderID, CustomerID) in order to satisfy queries 1 and 2?

Answer 1

This index will not help the second query, since it is not possible to first seek on the left-most column in the index.

Think about a phone book. First, try to find all the people with the last name Smith. Easy, right? Now, try to find all the people with the first name John. The way the “index” in a phone book works is LastName, FirstName. When you’re trying to find all the Johns, having them first sorted by LastName is not helpful at all – you still have to look through the whole phone book, because there will be a John Anderson and a John Zolti and everything in between.

To get the most out of all three queries, and assuming these are the only three query forms that are in use, I would potentially suggest an additional index:

create nonclustered index Example2
  On dbo.Orders(CustomerID) INCLUDE(OrderID)

(If OrderID is the primary key, you shouldn’t need to INCLUDE it.)

However, this should be tested against your workload, since indexes aren’t free – they do require additional disk space, and extra work to be maintained when you are running DML queries. And again, this assumes that the three queries listed in your question are the only shapes you use. If you have other queries with different output columns or different where clauses, all bets are off.