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Update locks are really not a separate kind of lock; they are a hybrid of shared and exclusive locks. They are acquired when SQL Server executes a data modi cation operation but rst, SQL Server needs to search the table to nd the resource that needs to be modi ed. Using query hints, a process can speci cally request update locks, and in that case, the update locks prevent the conversion deadlock situation presented in Figure 10-6 later in this chapter.
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Update locks provide compatibility with other current readers of data, allowing the process to later modify data with the assurance that the data hasn t been changed since it was last read. An update lock is not suf cient to allow you to change the data all modi cations require that the data resource being modi ed have an exclusive lock. An update lock acts as a serialization gate to queue future requests for the exclusive lock. (Many processes can hold shared locks for a resource, but only one process can hold an update lock.) So long as a process holds an update lock on a resource, no other process can acquire an update lock or an exclusive lock for that resource; instead, another process requesting an update or exclusive lock for the same resource must wait. The process holding the update lock can convert it into an exclusive lock on that resource because the update lock prevents lock incompatibility with any other processes. You can think of update locks as intent-to-update locks, which is essentially the role they perform. Used alone, update locks are insuf cient for updating data an exclusive lock is still required for actual data modi cation. Serializing access for the exclusive lock lets you avoid conversion deadlocks. Update locks are held until the end of the transaction or until they are converted to an exclusive lock. Don t let the name fool you: update locks are not just for UPDATE operations. SQL Server uses update locks for any data modi cation operation that requires a search for the data prior to the actual modi cation. Such operations include quali ed updates and deletes, as well as inserts into a table with a clustered index. In the latter case, SQL Server must rst search the data (using the clustered index) to nd the correct position at which to insert the new row. While SQL Server is only searching, it uses update locks to protect the data; only after it has found the correct location and begins inserting does it convert the update lock to an exclusive lock.
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Intent locks are not really a separate mode of locking; they are a quali er to the modes previously discussed. In other words, you can have intent shared locks, intent exclusive locks, and even intent update locks. Because SQL Server can acquire locks at different levels of granularity, a mechanism is needed to indicate that a component of a resource is already locked. For example, if one process tries to lock a table, SQL Server needs a way to determine whether a row (or a page) of that table is already locked. Intent locks serve this purpose. We discuss them in more detail when we look at lock granularity.
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SQL Server offers three additional lock modes: schema stability locks, schema modi cation locks, and bulk update locks. When queries are compiled, schema stability locks prevent other processes from acquiring schema modi cation locks, which are taken when a table s structure is being modi ed. A bulk update lock is acquired when the BULK INSERT command is executed or when the bcp utility is run to load data into a table. In addition, the bulk import operation must request this special lock by using the TABLOCK hint. Alternatively, the table owner can set the table option called table lock on bulk load to True, and then any bulk copy IN or BULK INSERT operation automatically requests a bulk update lock. Requesting
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