ASA SQL User's Guide
Using Transactions and Isolation Levels
Choosing isolation levels
Isolation levels 2 and 3 use a lot of locks and so good design is of particular importance for databases that make regular use of these isolation levels. When you must make use of serializable transactions, it is important that you design your database, in particular the indices, with the business rules of your project in mind. You may also improve performance by breaking large transactions into several smaller ones, thus shortening the length of time that rows are locked.
Although serializable transactions have the most potential to block other transactions, they are not necessarily less efficient. When processing these transactions, Adaptive Server Anywhere can perform certain optimizations that may improve performance, in spite of the increased number of locks. For example, since all rows read must be locked whether or not they match the a search criteria, the database server is free to combine the operation of reading rows and placing locks.