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Transactions and locking are perhaps second only in importance to relations between tables. The integrity and performance of any database can benefit from the judicious use of locking and careful construction of transactions. Both are essential to creating databases that must execute a large number of commands concurrently.
Transactions group SQL statements into logical units of work. You may end each by either rolling back any changes you have made or by committing these changes and so making them permanent.
Transactions are essential to data recovery in the event of system failure. They also play a pivotal role in interweaving statements from concurrent transactions.
To improve performance, multiple transactions must be executed concurrently. Each transaction is composed of component SQL statements. When two or more transactions are to be executed concurrently, the database server must schedule the execution of the individual statements. Concurrent transactions have the potential to introduce new, inconsistent results that could not arise were these same transactions executed sequentially.
Many types of inconsistencies are possible, but four typical types are particularly important because they are mentioned in the ISO SQL/2003 standard and the isolation levels are defined in terms of them.
Dirty read One transaction reads data modified, but not yet committed, by another.
Non-repeatable read A transaction reads the same row twice and gets different values.
Phantom row A transaction selects rows, using a certain criterion, twice and finds new rows in the second result set.
Lost update One transaction's changes to a row are completely lost because another transaction is allowed to save an update based on earlier data.
A schedule is called serializable whenever the effect of executing the statements according to the schedule is the same as could be achieved by executing each of the transactions sequentially. Schedules are said to be correct if they are serializable. A serializable schedule will cause none of the above inconsistencies.
Locking controls the amount and types of interference permitted. SQL Anywhere provides you with four levels of locking: isolation levels 0, 1, 2, and 3. At the highest isolation, level 3, SQL Anywhere guarantees that the schedule is serializable, meaning that the effect of executing all the transactions is equivalent to running them sequentially.
Unfortunately, locks acquired by one transaction may impede the progress of other transactions. Because of this problem, lower isolation levels are desirable whenever the inconsistencies they may allow are tolerable. Increased isolation to improve data consistency frequently means lowering the concurrency, the efficiency of the database at processing concurrent transactions. You must frequently balance the requirements for consistency against the need for performance to determine the best isolation level for each operation.
Conflicting locking requirements between different transactions may lead to blocking or deadlock. SQL Anywhere contains mechanisms for dealing with both these situations, and provides you with options to control them.
Transactions at higher isolation levels do not, however, always impact concurrency. Other transactions will be impeded only if they require access to locked rows. You can improve concurrency through careful design of your database and transactions. For example, you can shorten the time that locks are held by dividing one transaction into two shorter ones, or you might find that adding an index allows your transaction to operate at higher isolation levels with fewer locks.
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