Database Resource Manager, an Oracle Database Enterprise Edition option, is exceedingly useful for maintaining an Oracle Application Express instance. Using Resource Manager you can limit the types of system resources (
LOGICAL_READS_PER_CALL) available to each user request. By setting up multiple resource profiles, with automatic switches between the profiles, as a user request exceeds a certain threshold you can reduce the CPU available to that transaction. For example you may define a default profile APEX_HIGH which can utilize 70% of CPU for 10 seconds, APEX_MEDIUM with 8% CPU for 120 seconds, APEX_LOW with 2% CPU for 1800 seconds, and OTHERS with 20% CPU. When a specific transaction exceeds 10 seconds, it is switched to the APEX_MEDIUM profile which drastically reduces the CPU available. If the transaction switches to APEX_LOW and exceeds 1800 seconds then you can cancel the SQL or kill the session. This approach ensures that one specific user or poorly written application cannot monopolize the CPU causing other users and applications to respond poorly.
Instance administrators can define Resource Manager profiles for individual workspaces.
"Configuring Workspace Isolation Attributes" and "Configuring Instance-Level Workspace Isolation Attributes" in Oracle Application Express Administration Guide
"Managing Resources with Oracle Database Resource Manager" in Oracle Database Administrator's Guide