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SQL Anywhere 12.0.0 » SQL Anywhere Server - SQL Usage » Using transactions and isolation levels

 

Choosing isolation levels

The choice of isolation level depends on the kind of task an application is performing. This section gives some guidelines for choosing isolation levels.

To 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 compared to one that searches many large tables and therefore 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, you likely want 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 you 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 to calculate the estimate. For this reason, the transfer is less likely to delay other transactions.

SQL Anywhere provides four isolation levels: 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.

If you have enabled snapshot isolation for a database, then three additional isolation levels are available: snapshot, statement-snapshot, and readonly-statement-snapshot.

 Choosing a snapshot isolation level

Serializable schedules
Typical transactions at various isolation levels
Improving concurrency at isolation levels 2 and 3
Reducing the impact of locking