Hugo Kornelis talks about a fairly recent property in execution plans:
There are two operators that read from the SalesOrderDetail table (or from indexes on that table). The top left operator is an Index Seek on one of the nonclustered indexes on SalesOrderDetail, and on the bottom input of the Nested Loops operator is a Clustered Index Scan that scans the clustered index on the same table.
So, now what? Which of the two is in this case the problem? Is each doing exactly 625 logical reads? Is one doing 50 and the other 1200? For the longest time, there was no way to find out. Sometimes you could make an educated guess by looking at the rest of the execution plan. Sometimes you can get an idea by running other queries with similar plans and check their logical reads (like in this case, you could run the subquery by itself and that would work). But none of these methods are really satisfactory.
Read on to see how the SQL Server team has addressed this.