SQL Server Execution Plans - Red Gate Software

Chapter 5: Controlling Execution Plans with Hints
The performance penalty comes from the fact that there was a single scan and two reads
on the ProductModel table in the original query. Use of the Nested Loops join changed
the behavior to a single scan with 1,009 reads. Reading more data will have an impact on
the performance of queries.
Summary
While the optimizer makes very good decisions most of the time, it may sometimes make
less than optimal choices. Taking control of the queries using Table, Join and Query hints,
when appropriate, can often be the right choice. However, remember that the data in
your database is constantly changing. Any choices you force on the optimizer through
hints today, to achieve whatever improvement you're hoping for, may become a major
pain in the future.
If you decide to use hints, test them prior to applying them, and remember to document
their use in some manner so that you can come back and test them again periodically as
your database grows. As Microsoft releases patches and service packs, the behavior of the
optimizer can change. Be sure to retest any queries using hints after an upgrade to your
server. I intentionally demonstrated cases where the query hints hurt as well as help,
as this simply reflects reality. Use of these hints should be a last resort, not a standard
method of operation.
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