ASA SQL User's Guide
Using Transactions and Isolation Levels
The choice of isolation level depends on the kind of task an application is carrying out. This section gives some guidelines for choosing isolation levels.
When you choose an appropriate isolation level you must balance the need for consistency and accuracy with the need for concurrent transactions to proceed unimpeded. If a transaction involves only one or two specific values in one table, it is unlikely to interfere as much with other processes as one which searches many large tables and may need to lock many rows or entire tables and may take a very long time to complete.
For example, if your transactions involve transferring money between bank accounts or even checking account balances, you will likely want to do your utmost to ensure that the information you return is correct. On the other hand, if you just want a rough estimate of the proportion of inactive accounts, then you may not care whether your transaction waits for others or not and indeed may be willing to sacrifice some accuracy to avoid interfering with other users of the database.
Furthermore, a transfer may affect only the two rows which contain the two account balances, whereas all the accounts must be read in order to calculate the estimate. For this reason, the transfer is less likely to delay other transactions.
Adaptive Server Anywhere provides four levels of isolation: levels 0, 1, 2, and 3. Level 3 provides complete isolation and ensures that transactions are interleaved in such a manner that the schedule is serializable.
Serializable schedules
Typical transactions at various isolation levels
Improving concurrency at isolation levels 2 and 3
Reducing the impact of locking