MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 14: Transactions

Setting the Transaction Isolation Level

To set the transaction isolation level, an application uses the SQL_ATTR_TXN_ISOLATION connection attribute. If the data source does not support the requested isolation level, the driver or data source can set a higher level. To determine what transaction isolation levels a data source supports and what the default isolation level is, an application calls SQLGetInfo with the SQL_TXN_ISOLATION_OPTION and SQL_DEFAULT_TXN_ISOLATION options, respectively.

Higher levels of transaction isolation offer the most protection for the integrity of database data. Serializable transactions are guaranteed to be unaffected by other transactions and therefore guaranteed to maintain database integrity.

However, a higher level of transaction isolation can cause slower performance because it increases the chances that the application will have to wait for locks on data to be released. An application may specify a lower level of isolation to increase performance in the following cases:

When it can be guaranteed that no other transactions exist that might interfere with an application's transactions. This situation occurs only in limited circumstances, such as when one person in a small company maintains dBASE files containing personnel data on one computer and does not share these files.
When speed is more critical than accuracy and any errors are likely to be small. For example, suppose that a company makes many small sales and that large sales are rare. A transaction that estimates the total value of all open sales might safely use the Read Uncommitted isolation level. Although the transaction would include orders in the process of being opened or closed that are subsequently rolled back, these would tend to cancel each other out and the transaction would be much faster because it is not blocked each time it encounters such an order.

For more information, see the section "Optimistic Concurrency," later in this chapter.