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SQL: A Beginner s Guide
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CD_NAME Famous Blue Raincoat Transaction 1 begins Court and Spark Past Light
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STOCK 13 42 22
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CD_NAME Famous Blue Raincoat Transaction 1 updates table Court and Spark Past Light
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STOCK 12 26 11 Transaction 2 begins and reads table
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CD_NAME Transaction 1 rolls back update Famous Blue Raincoat Court and Spark Past Light
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Transaction 2 takes action based on dirty read
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Concurrent transactions resulting in a dirty read
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Nonrepeatable Reads The next phenomenon that can occur when concurrent transactions are initiated is the nonrepeatable read. The nonrepeatable read can occur when one transaction reads data from a table, another transaction then updates that data, and the first transaction rereads the data, only to discover that the data has changed. As a result, the first read is not repeatable. Let s take a look at Figure 16-3 to better understand this concept. When Transaction 1 is initiated, it reads the data in the table. At that point, the transaction might be involved in other processes or is waiting for a response from a user. For example, the user might receive a call from a manager who is trying to find out how many copies of a particular CD are in stock. The user checks for that information. The manager then puts the user on hold for a short time, so the user must wait to complete the transaction. During that time, Transaction 2 is initiated and it updates the table. After the update, Transaction 1 again reads the data (the manager returns to the phone) and finds different information from the first read, resulting in a nonrepeatable read. Phantom Reads The last phenomenon that we ll look at is the phantom read. Although similar to the nonrepeatable read, the phantom read has some subtle differences, which are factors when trying to determine an isolation level. A phantom read can occur when a transaction reads a table based on some sort of search condition, then a second transaction updates the data in the table, and then the first transaction attempts to reread the data, only this time different rows are returned because of how the search condition is defined. To clarify this, let s take a look at Figure 16-4.
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Managing SQL Transactions
CD_NAME Transaction 1 begins and reads table Famous Blue Raincoat Court and Spark Past Light
STOCK 13 42 22
CD_NAME Famous Blue Raincoat Court and Spark Past Light
STOCK 12 26 11 Transaction 2 begins and updates table
CD_NAME Transaction 1 reads values different from first read Famous Blue Raincoat Court and Spark Past Light
STOCK 12 26 11
Figure 16-3
Concurrent transactions resulting in a nonrepeatable read
When Transaction 1 is initiated, it reads the data in the table by executing a SELECT statement that queries the data. The statement includes a WHERE clause that returns only those rows with a STOCK value greater than 20. That means that the Court and Spark row and the Past Light row are returned. After Transaction 1 retrieves (reads) the data, Transaction 2 begins and updates the table. Now when Transaction 1 rereads the data (using the same search criteria), only the Famous Blue Raincoat row is returned because it is now the only row with a STOCK value greater than 20. As a result, the transaction has experienced a phantom read; the rows that it s reading are not the same rows as it saw earlier.
Choosing an Isolation Level
Now that you have an overview of the type of data phenomena that you can run into when you have concurrent transactions, you should be better equipped to choose an isolation level for your transaction. The important point to remember is that the more restrictive the isolation level, the more types of phenomena you can eliminate. Let s take a look at the READ UNCOMMITTED isolation level, the least restrictive of the four levels. A transaction that s configured with this option may experience any of the data phenomena that we ve looked at (dirty read, nonrepeatable read, and phantom read). As you can imagine, this is normally not a desirable state. In fact, if you define a transaction with the READ UNCOMMITTED option, the transaction cannot include statements that modify data. These transactions, by default, have a READ ONLY access level, and you cannot specify a
SQL: A Beginner s Guide
Transaction 1 begins and returns rows based on search condition
CD_NAME Famous Blue Raincoat Court and Spark Past Light
STOCK 13 42 22
CD_NAME Famous Blue Raincoat Court and Spark Out of Africa
STOCK 42 13 11 Transaction 2 begins and updates table
Transaction 1 returns different rows based on same search condition
CD_NAME Famous Blue Raincoat Court and Spark Out of Africa
STOCK 42 13 11
Figure 16-4
Concurrent transactions resulting in a phantom read
READ WRITE level. (All other isolation levels have a default access level of READ WRITE.) You should use the READ UNCOMMITTED isolation level only for transactions that generate approximate information, such as some types of statistical data in which the results are not critical in terms of precise accuracy. The READ COMMITTED isolation level is only slightly more restrictive than READ UNCOMMITTED. The READ COMMITTED option prevents dirty reads, but nonrepeatable reads and phantom reads can still occur. The next option, REPEATABLE READ, is even more restrictive than READ COMMITTED. It prevents dirty reads and nonrepeatable reads, but does not prevent phantom reads. The only option that prevents all three types of data anomalies is SERIALIZABLE. A transaction that is defined with the SERIALIZABLE isolation level fully isolates that transaction from all other transactions. As a result, the transaction is said to be serializable, meaning that it interacts with concurrent transactions in a way that orders transactions sequentially so that one transaction cannot impact the other. This does not mean that one transaction must close before another can open, but it does mean that the results of those transactions have to be the same as the results of operations that do operate one at a time. As long as no serializable transaction can influence another serializable transaction, the transactions are in conformance to the SERIALIZABLE isolation level.
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