Effects of Isolation Levels

You cannot set the transaction isolation level in a database that does not have logging. Every retrieval in such a database occurs as a Dirty Read.

The data retrieved from a BYTE or TEXT column can vary, depending on the transaction isolation level. Under Dirty Read or Committed Read levels of isolation, a process can read a BYTE or TEXT column that is either deleted (if the delete is not yet committed) or in the process of being deleted. Under these isolation levels, deleted data is readable under certain conditions. For information about these conditions, see the Informix® Administrator's Guide.

When you use DB-Access, as you use higher levels of isolation, lock conflicts occur more frequently. For example, if you use Cursor Stability, more lock conflicts occur than if you use Committed Read.

Using a scroll cursor in an ESQL/C transaction, you can force consistency between your temporary table and the database table either by setting the level to Repeatable Read or by locking the entire table during the transaction.

If you use a scroll cursor WITH HOLD in a transaction, you cannot force consistency between your temporary table and the database table. A table-level lock or locks that are set by Repeatable Read are released when the transaction is completed, but the scroll cursor with hold remains open beyond the end of the transaction. You can modify released rows as soon as the transaction ends, but retrieved data in the temporary table might be inconsistent with the actual data.
Attention: Do not use nonlogging tables within a transaction. If you need to use a nonlogging table within a transaction, either set the isolation level to Repeatable Read or else lock the table in Exclusive mode to prevent concurrency problems.