SQL Server 2008 :: MERGE Statement - Cannot Filter Set Of Rows In Target Table

I have a table:

declare tableName table
(
uniqueid int identity(1,1),
id int,
starttime datetime2(0),
endtime datetime2(0),
parameter int
)

A stored procedure has new set of values for a given id. Sometimes the startime and endtime are the same, in which case I update the value of parameter. Sometimes I add a new time range (insert statement), and sometimes I delete a time range (delete statement).

I had a question on merge, with insert, delete and update and I got that resolved. However I have a different question regarding performance of the merge statement.

If my target table has hundreds of millions of records and I want to delete/update/insert a handful of records, will SQL server scan the entire target table? I can't have:

merge ( select * from tableName where id = 10 ) as target
using ...

and I can't have:

merge tableName as target
using [my query] as source on
source.id = target.id and
source.starttime = target.startime and
source.endtime = target.endtime
where target.id = 10
...

This means I cannot filter the set of rows in the target table to a handful of records where id = 10.