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Reference : Designing ODBC Applications for Performance Optimization : Managing Connections and Updates : Managing Commits in Transactions

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Managing Commits in Transactions
Committing data is extremely disk I/O intensive and slow. If the driver can support transactions, always turn autocommit off.
What does a commit actually involve? The database server must flush back to disk every data page that contains updated or new data. This is not a sequential write but a searched write to replace existing data in the table. By default, autocommit is on when connecting to a data source. Autocommit mode usually impairs system performance because of the significant amount of disk I/O needed to commit every operation.
Some database servers do not provide an Autocommit mode. For this type of server, the ODBC driver must explicitly issue a COMMIT statement and a BEGIN TRANSACTION for every operation sent to the server. In addition to the large amount of disk I/O required to support Autocommit mode, a performance penalty is paid for up to three network requests for every statement issued by an application.
Although using transactions can help application performance, do not take this tip too far. Leaving transactions active can reduce throughput by holding locks on rows for long times, preventing other users from accessing the rows. Commit transactions in intervals that allow maximum concurrency.