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Versioning does not completely eliminate the possibility of deadlocks within the database. The two transactions in Figure 12-13, with their interleaved attempts to update two different tables, each in a different order, will still produce problems, even for a versioning scheme. However, for workloads with a mix of database READ operations and database UPDATE operations, versioning can significantly reduce the locking and lock timeouts or deadlocks associated with shared locks.
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The advantage of a versioning architecture is that, under the right circumstances, it can significantly increase the number of concurrent transactions that can execute in parallel. Concurrent execution is becoming more and more important in large DBMS installations, especially those that support web sites that may have thousands or tens of thousands of concurrent users. Versioning is also becoming more useful as the number of processors on typical DBMS server computer systems increases. Servers containing 16 or more processors are becoming increasingly common, and large DBMS servers may support 64 or more processors in a symmetric multiprocessing (SMP) configuration. These servers can actually execute many database-access applications in parallel by spreading the workload out over many processors. The disadvantage of a versioning architecture is the internal DBMS overhead that it creates. One obvious overhead is the added memory and disk requirement of storing two or more copies of rows that are being updated. In practice, a more serious overhead is the memory management required to allocate memory for each temporary copy of a row as it is needed (potentially thousands of times per second), and then releasing the memory to be reused when the older copies of the row are no longer needed. An additional overhead is keeping track of which transactions should see which copies of which rows. Implicitly, a versioning architecture is based on the underlying assumption that most concurrent transactions will tend not to interfere with one another. If this assumption proves accurate (i.e., if concurrently executing transactions mostly access and update different rows, or if the transaction workload is dominated by READ operations rather than UPDATEs), then the added overhead of the versioning scheme will be more than offset by a significant boost in the amount of parallel work that can be performed. If the assumption proves inaccurate (i.e., if concurrently executing transactions tend to access and update the same rows), then the overhead of the versioning technique will tend to become very high, swamping the concurrency gains.
PART III
Summary
This chapter described the transaction mechanism provided by the SQL language: A transaction is a logical unit of work in a SQL-based database. It consists of a sequence of SQL statements that are effectively executed as a single unit by the DBMS. The SET TRANSACTION and START transaction statements can be used to set the isolation level and access level of transactions. The SAVEPOINT statement creates an intermediate recovery point within a transaction. The RELEASE SAVEPOINT statement removes a savepoint and releases any resources it is holding. The COMMIT statement signals successful completion of a transaction, making all of its database modifications permanent. The ROLLBACK statement asks the DBMS to abort a transaction, backing out all of its database modifications.
Part III:
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Transactions are the key to recovering a database after a system failure; only transactions that were committed at the time of failure remain in the recovered database. Transactions are the key to concurrent access in a multiuser database. A user or program is guaranteed that its transaction will not be interfered with by other concurrent transactions. Occasionally, a conflict with another concurrently executing transaction may cause the DBMS to roll back a transaction through no fault of its own. An application program that uses SQL must be prepared to deal with this situation if it occurs. The subtleties of transaction management, and their impact on DBMS performance, are one of the more complex areas of using and operating a large production database. This is also an area where major DBMS brands diverge in their capabilities and tuning options. Many DBMS brands use locking techniques to handle concurrent transactions. For these products, adjustments to the locking parameters and explicit locking statements allow you to tune transaction-processing performance. An alternative versioning technique for handling concurrent transactions is supported by some current products. For DBMS products that use versioning, adjustments to the depth of the versioning scheme and to the transaction mix itself are the keys to performance tuning.
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