Query optimizer prefers index scan over index seek

Question

I am using AdventureWorks2012 database.

I created the following index on Sales.SalesOrderHeader table

create index i1 on 
sales.salesorderheader(purchaseordernumber,salespersonid)
   include(orderdate,shipdate)
   where purchaseordernumber is not null
   and salespersonid is not null

When I run the below query:

select 
    PurchaseOrderNumber,
    OrderDate,
    ShipDate,
    SalesPErsonId

from sales.salesorderheader
where purchaseordernumber like '%po5%' and salespersonid is not null

I expect an Index seek not an index scan, as all of the columns which are part of the query are already part of the index.

I read somewhere that if the query optimizer feels that index scan is cheaper than index seek, then we will index scan. But, I don't understand why it's happening in this case. We don't have a lookup at all.

Can someone explain me please.

Answer 1

An index works just like a phonebook. Where would you look for %po5% in a phonebook? You'd have to read the entire phonebook, because you don't know the first character.

That's why SQL Server scans the entire index. It does not have enough information to seek.