#ATOMIC TRANSACTION DEFINITION UPDATE#
#ATOMIC TRANSACTION DEFINITION FREE#

Conversely, if you are sure you won't need to roll back to a particular savepoint again, it can be released, so the system can free some resources.

All the transaction's database changes between defining the savepoint and rolling back to it are discarded, but changes earlier than the savepoint are kept.Īfter rolling back to a savepoint, it continues to be defined, so you can roll back to it several times. After defining a savepoint with SAVEPOINT, you can if needed roll back to the savepoint with ROLLBACK TO. Savepoints allow you to selectively discard parts of the transaction, while committing the rest.

It's possible to control the statements in a transaction in a more granular fashion through the use of savepoints. Check the documentation for the interface you are using. Some client libraries issue BEGIN and COMMIT commands automatically, so that you might get the effect of transaction blocks without asking. A group of statements surrounded by BEGIN and COMMIT is sometimes called a transaction block. If you do not issue a BEGIN command, then each individual statement has an implicit BEGIN and (if successful) COMMIT wrapped around it. PostgreSQL actually treats every SQL statement as being executed within a transaction. If, partway through the transaction, we decide we do not want to commit (perhaps we just noticed that Alice's balance went negative), we can issue the command ROLLBACK instead of COMMIT, and all our updates so far will be canceled. So our banking transaction would actually look like: In PostgreSQL, a transaction is set up by surrounding the SQL commands of the transaction with BEGIN and COMMIT commands. The updates made so far by an open transaction are invisible to other transactions until the transaction completes, whereupon all the updates become visible simultaneously. So transactions must be all-or-nothing not only in terms of their permanent effect on the database, but also in terms of their visibility as they happen. For example, if one transaction is busy totalling all the branch balances, it would not do for it to include the debit from Alice's branch but not the credit to Bob's branch, nor vice versa. A transactional database guarantees that all the updates made by a transaction are logged in permanent storage (i.e., on disk) before the transaction is reported complete.Īnother important property of transactional databases is closely related to the notion of atomic updates: when multiple transactions are running concurrently, each one should not be able to see the incomplete changes made by others. For example, if we are recording a cash withdrawal by Bob, we do not want any chance that the debit to his account will disappear in a crash just after he walks out the bank door. We also want a guarantee that once a transaction is completed and acknowledged by the database system, it has indeed been permanently recorded and won't be lost even if a crash ensues shortly thereafter. A transaction is said to be atomic: from the point of view of other transactions, it either happens completely or not at all. Grouping the updates into a transaction gives us this guarantee. We need a guarantee that if something goes wrong partway through the operation, none of the steps executed so far will take effect. Nor would Alice long remain a happy customer if she was debited without Bob being credited. It would certainly not do for a system failure to result in Bob receiving $100.00 that was not debited from Alice. Our bank's officers will want to be assured that either all these updates happen, or none of them happen. The details of these commands are not important here the important point is that there are several separate updates involved to accomplish this rather simple operation. WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Bob')

#ATOMIC TRANSACTION DEFINITION UPDATE#

UPDATE branches SET balance = balance + 100.00 UPDATE accounts SET balance = balance + 100.00

WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Alice') UPDATE branches SET balance = balance - 100.00 UPDATE accounts SET balance = balance - 100.00 Simplifying outrageously, the SQL commands for this might look like: Suppose that we want to record a payment of $100.00 from Alice's account to Bob's account. The intermediate states between the steps are not visible to other concurrent transactions, and if some failure occurs that prevents the transaction from completing, then none of the steps affect the database at all.įor example, consider a bank database that contains balances for various customer accounts, as well as total deposit balances for branches. The essential point of a transaction is that it bundles multiple steps into a single, all-or-nothing operation. Transactions are a fundamental concept of all database systems.

Atomic transaction definition