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

Chapter 14: Transactions and Locks
level (not check every row or page lock on the table) to determine if a transaction can safely lock the
entire table. Intent locks come in three different varieties:
❑ Intent shared lock: A shared lock has been, or is going to be, established at some lower point in
the hierarchy. For example, a page is about to have a page-level shared lock established on it.
This type of lock applies only to tables and pages.
❑ Intent exclusive lock: This is the same as intent shared, but with an exclusive lock about to be
placed on the lower-level item.
❑ Shared with intent exclusive lock: A shared lock has been, or is about to be, established lower
down the object hierarchy, but the intent is to modify data, so it will become an intent exclusive
at some point.
Schema Locks
These come in two flavors:
❑ Schema modification lock (Sch-M): A schema change is being made to the object. No queries or
other CREATE, ALTER, or DROP statements can be run against this object for the duration of the
Sch-M lock.
❑ Schema stability lock (Sch-S): This is very similar to a shared lock; this lock’s sole purpose is to
prevent a Sch-M, since there are already locks for other queries (or CREATE, ALTER, or DROP
statements) active on the object. This is compatible with all other lock types.
Bulk Update Locks
A bulk update lock (BU) is really just a variant of a table lock with one little (but significant) difference. Bulk
update locks will allow parallel loading of data — that is, the table is locked from any other normal activity
(T-SQL statements), but multiple BULK INSERT or bcp operations can be performed at the same time.
Lock Compatibility
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The table that follows shows the compatibility of the resource lock modes (listed in increasing lock
strength). Existing locks are shown by the columns, requested locks by the rows:
IS S U IX SIX X
Intent Shared (IS) YES YES YES YES YES NO
Shared (S) YES YES YES NO NO NO
Update (U) YES YES NO NO NO NO
Intent Exclusive (IX) YES NO NO YES NO NO
Shared with Intent Exclusive (SIX) YES NO NO NO NO NO
Exclusive (X) NO NO NO NO NO NO