Are you expecting the knowledge engineer (the person writing a rules base) to be a programmer ? A Perl programmer ?

What degree of complexitiy of (real-world) problem do you expect to be able to solve ?

For a user, a table isn't a complex concept, but inputing it might be unless the system is designed to cope at the beginning. Writing a rule such as (in my pseudo code) :

[ID in (A B C)]
=>
[D = rows from TABLE_D where KEY=ID]
[download]

could be difficult otherwise.

Re. The rule efficiency, you are right, this is the hard part.
To some extent the responsibility lies with the knowledge engineer (read ruleset creator), since you can write bad (==slow) programs/algorithms in any language.

Giving the engineer tools (such as the priority / weighting mechanism you mention can help, as can other options such as choice of search mechanism and direction.

In my experience, a lot of knowledge engineer time is spent tuning the knowledge base, and the system should be proactive in helping that process, providing timing / coverage analysis.
--
Brovnik