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SQL Server 2008 supports two kinds of key locks, and which one it uses depends on the isolation level of the current transaction. If the isolation level is Read Committed, Repeatable Read, or Snapshot, SQL Server tries to lock the actual index keys accessed while processing the query. With a table that has a clustered index, the data rows are the leaf level of the index, and you see key locks acquired. If the table is a heap, you might see key locks for the nonclustered indexes and row locks for the actual data. If the isolation level is Serializable, the situation is different. We want to prevent phantoms, so if we have scanned a range of data within a transaction, we need to lock enough of the table
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to make sure no one can insert a value into the range that was scanned. For example, we can issue the following query within an explicit transaction in the AdventureWorks2008 database:
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BEGIN TRAN SELECT * FROM Sales.SalesOrderHeader WHERE CustomerID BETWEEN 100 and 110;
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When you use Serializable isolation, locks must be acquired to make sure no new rows with CustomerID values between 100 and 110 are inserted before the end of the transaction. Much older versions of SQL Server (prior to 7.0) guaranteed this by locking whole pages or even the entire table. In many cases, however, this was too restrictive more data was locked than the actual WHERE clause indicated, resulting in unnecessary contention. SQL Server 2008 uses the key-range locks mode, which is associated with a particular key value in an index and indicates that all values between that key and the previous one in the index are locked. The AdventureWorks2008 database includes an index on the Person table with the LastName column as the leading column. Assume that we are in TRANSACTION ISOLATION LEVEL SERIALIZABLE and we issue this SELECT statement inside a user-de ned transaction:
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SELECT * FROM Person.Person WHERE LastName BETWEEN 'Freller' AND 'Freund';
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If Fredericksen, French, and Friedland are sequential leaf-level index keys in an index on the LastName column, the second two of these keys (French and Friedland) acquire key-range locks (although only one row, for French, is returned in the result set). The key-range locks prevent any inserts into the ranges ending with the two key-range locks. No values greater than Fredericksen and less than or equal to French can be inserted, and no values greater than French and less than or equal to Friedland can be inserted. Note that the key-range locks imply an open interval starting at the previous sequential key and a closed interval ending at the key on which the lock is placed. These two key-range locks prevent anyone from inserting either Fremlich or Frenkin, which are in the range speci ed in the WHERE clause. However, the key-range locks would also prevent anyone from inserting Freedman (which is greater than Fredericksen and less than French), even though Freedman is not in the query s speci ed range. Key-range locks are not perfect, but they do provide much greater concurrency than locking whole pages or tables, while guaranteeing that phantoms are prevented. There are nine types of key-range locks, and each has a two-part name: the rst part indicates the type of lock on the range of data between adjacent index keys, and the second part indicates the type of lock on the key itself. These nine types of key-range locks are described in Table 10-4.
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TABLE 10-4
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Transactions and Concurrency
Types of Key-Range Locks
Description
Shared lock on the range between keys; shared lock on the key at the end of the range Shared lock on the range between keys; update lock on the key at the end of the range Exclusive lock to prevent inserts on the range between keys; no lock on the keys themselves Exclusive lock on the range between keys; exclusive lock on the key at the end of the range Conversion lock created by S and RangeIn_Null lock Conversion lock created by U and RangeIn_Null lock Conversion of X and RangeIn_Null lock Conversion of RangeIn_Null and RangeS_S lock Conversion of RangeIn_Null and RangeS_U lock
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