Automatic undo management depends upon the UNDO_RETENTION parameter, which defines how long Oracle should try to keep committed transactions in UNDO segments.However, the UNDO_RETENTION parameter is only a suggestion.
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For documentation on SAVEPOINT transactions, please see Perform a bulk insert of the given list of mapping dictionaries.
The target database(s) and associated drivers must support SQL SAVEPOINT for this method to function correctly.
web2py comes with a Database Abstraction Layer (DAL), an API that maps Python objects into database objects such as queries, tables, and records.
The DAL dynamically generates the SQL in real time using the specified dialect for the database back end, so that you do not have to write SQL code or learn different SQL dialects (the term SQL is used generically), and the application will be portable among different types of databases.
With this setting, transactions are never automatically started.
All transactions must be explicitly started by executing the statement may be used to explicitly start a transaction if required, typically to allow several updates to be grouped together within a single transaction for consistency, as already described.
The ORA-01555 is caused by Oracle read consistency mechanism.
If you have a long running SQL that starts at AM, Oracle ensures that all rows are as they appeared at AM, even if the query runs until noon!
An automatically started transaction is concluded by executing a statement is most commonly used in exception routines for handling error situations that are detected by the application during transaction build-up.
If a connection or program is terminated without requesting a for the current transaction, the system will abort the transaction.
The following sections explain how to start, end and optimize transactions.