PERMISSIONS

xpg contains a mechanism to limit the access of users and groups of users to different operations normally allowed by xpg, on a relation or database basis.  The operations are:  
  - Reading (and viewing) a relation;
  - Appending tuples to a relation;
  - Modifying (writing into) a relation - this includes the Append, Replace and Delete commands.

When the user tries to commit a limited-access command, xpg does nothing but popping-up an error message.  Also, the Execute Dialog cannot be accessed if any of the above permissions was limited.  This is because the Execute Dialog gives the user the power to execute any command.  
The access control is in effect also while playing scripts.

Maintaining the xpg permission rules is the job of the system administrators.  There is no way for the user to define permission rules, even for his/her own database.  Despite this, unless changed by your system administrators, xpg gives each user full access to his/her own database, overriding any permission rules.

The xpg access control system is similar to the Postgres ACL system, available from version 4.1.  Despite this similarity, xpg does not use the ACL system to implement its own access control system.  This makes xpg's mechanism usable also with versions of Postgres older than 4.1.