Long transaction error caused by a SELECT.. WHERE EXISTS...

Hola Informixers: I'm getting confused about the following case. I've got a Long Transaction error when running a simple SELECT with an EXISTS clause. The problem is that my OnLine 5.01 writes all rows satisfying the EXISTS condition to the logical logs. How comes? Why aren't they written to an ordinary temp table? As long as there is no data modification, I just cant understand why the logical logs are involved. In the customer tech service they told me that this is a feature. Ok, I can rewrite the select avoiding the EXISTS clause or increase my logical logs, but the point is to understand the role of logical logs. Any help will be appreciated.