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Are there any performance implications to using transactions Indeed there are. The DBMS must carefully log the effects of every SQL statement inside a transaction so that changes can be rolled back when needed. And it is possible to fill the memory areas set aside for tracking transactions. To counter this, SQL programmers sometimes break processes that apply lots of changes to the database into multiple transactions using commits. Check your DBMS documentation to see if there are restrictions on using commits during program loops, particularly when cursors are open.
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Earlier in the chapter, you state that statements such as the SELECT, DELETE, UPDATE, and CREATE TABLE statements must be executed within the context of a transaction. However, we have not been using transactions in the examples and Try This exercises throughout the book. When are transactions used Throughout the book, we have been using interactive SQL (direct invocation) to communicate with the database. Most of the SQL statements that we have been executing within this environment have been done within the context of a transaction, even though you weren t aware of that happening. For most SQL implementations, each SQL statement is considered its own transaction, a processing mode that is often called autocommit. When you execute the statement, a transaction is initiated. If the statement is successful, any changes made are committed to the database and the transaction is terminated, in much the same way as if you had executed a COMMIT statement. If the statement is not successful, the changes are rolled back, the database is returned to the state it was in when the statement was first executed, and the transaction is terminated, as though you executed a ROLLBACK statement. Although interactive SQL tends to treat each statement as its own transaction, you can usually execute transaction-related statements in this environment. However, which statements you can execute and what options they support vary from product to product, so make sure you check the documentation. In general, it is not necessary to specifically define a transaction in interactive SQL.
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In this Try This exercise you will create several transactions that execute statements against the INVENTORY database. For each transaction, you will explicitly start the transaction and execute one or more SQL statements. For this exercise, you will work with the COMMIT statement and ROLLBACK statement in separate transactions because you re working with directly invoked SQL (in your client application). However, if you were initiating transactions from within an application programming language, you would no doubt be using COMMIT and ROLLBACK together in some sort of conditional structure. In that way, certain results would cause the transaction to roll back, and other results would cause the transaction to commit, depending on how you set up the conditions in the programming language. However, for this exercise, we keep them separate so that you can effectively run through these steps. You can download the Try_This_16.txt file, which contains the SQL statements used in this exercise.
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1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. The first transaction that you ll create uses a START TRANSACTION statement to set the
isolation level to READ UNCOMMITTED, retrieves information from the ARTISTS table, and then commits the transaction. Enter and execute the following SQL transaction:
16:
Managing SQL Transactions
START TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; SELECT * FROM ARTISTS; COMMIT;
The transaction should return all the rows and columns from the ARTISTS table.
3. The next transaction that you ll create also uses a START TRANSACTION statement to
set the isolation level. But this time you ll be setting the level to SERIALIZABLE. Because SERIALIZABLE is the default, you aren t required to define it; however, for the purposes of this exercise, we re going to include it. After you start the transaction, you ll attempt to update the COMPACT_DISCS table by increasing the IN_STOCK value by 2 for all rows with a LABEL_ID value equal to 832. After the UPDATE statement, you ll roll back the transaction so that no data is modified in the database. Enter and execute the following SQL transaction:
START TRANSACTION ISOLATION LEVEL SERIALIZABLE; UPDATE COMPACT_DISCS SET IN_STOCK = IN_STOCK + 2 WHERE LABEL_ID = 832; ROLLBACK;
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