I have a long running query that I start the night/day before and work with it during the day and the next. If I let it run, and work with the grid and data soon after it returns, there are no problems; but if I run it and let it sit idle (for instance over night and come back the next day to work with the grid and data) it can give me the error "Cannot perform this operation on a closed dataset".
If I click reconnect in the Session menu, the grid refreshs the information is lost and the datagrid cleared.
Timeout and loss of session to database.
When there is a time out and the session is lost, the data may not be accessible. Please try not to leave session idle to long and make sure there is no timeout setting for the database (this is set on the Oracle end and not Toad).