Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 14: Transactions and Locks
Lock Escalation and Lock Effects on Performance
Escalation is all about recognizing that maintaining a finer level of granularity (say, a row lock instead of
a page lock) makes a lot of sense when the number of items being locked is small. However, as we get
more and more items locked, the overhead associated with maintaining those locks actually hinders performance.
It can cause the lock to be in place longer (thus creating contention issues — the longer the
lock is in place, the more likely that someone will want that particular record). When you think about
this for a bit, you’ll realize there’s probably a balancing act to be done somewhere, and that’s exactly
what the lock manager uses escalation to do.
When the number of locks being maintained reaches a certain threshold, the lock is escalated to the next
highest level and the lower-level locks do not have to be so tightly managed (freeing resources and helping
speed over contention).
Note that the escalation is based on the number of locks rather than the number of users. The importance
here is that you can single-handedly lock a table by performing a mass update — a row lock can
graduate to a page lock, which then escalates to a table lock. That means that you could potentially be
locking every other user out of the table. If your query makes use of multiple tables, it’s actually quite
possible to wind up locking everyone out of all of those tables.
While you certainly would prefer not to lock all the other users out of your object, there are times when
you still need to perform updates that are going to have that effect. There is very little you can do about
escalation other than to keep your queries as targeted as possible. Recognize that escalations will happen,
so make sure you’ve thought about what the possible ramifications of your query are.
Lock Modes
Beyond considering just what resource level you’re locking, you also should consider what lock mode
your query is going to acquire. Just as there are a variety of resources to lock, there are also a variety of
lock modes.
Some modes are exclusive of each other (which means they don’t work together). Some modes do nothing
more than essentially modify other modes. Whether modes can work together is based on whether
they are compatible (we’ll take a closer look at compatibility between locks later in this chapter).
Just as we did with lockable resources, let’s take a look at lock modes one by one.
Shared Locks
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This is the most basic type of lock there is. A shared lock is used when you need only to read the data —
that is, you won’t be changing anything. A shared lock wants to be your friend, as it is compatible with
other shared locks. That doesn’t mean that it still won’t cause you grief — while a shared lock doesn’t
mind any other kind of lock, there are other locks that don’t like shared locks.
Shared locks tell other locks that you’re out there. It’s the old, “Look at me! Ain’t I special?” thing. They
don’t serve much of a purpose, yet they can’t really be ignored. However, one thing that shared locks do
is prevent users from performing dirty reads.