The correct place for a sysadmin to limit memory usage would be in the ulimit settings the postmaster starts under. Of course, Neil's argument still holds in general: anyone who can write arbitrary queries is not going to have any difficulty in soaking up unreasonable amounts of resources. Trying to restrict that would probably make the system less useful rather than more so.
I'm not sure if I agree that there's no potential for implementing better resource limits/quotas in PG in the future, I was just pointing out that it would require a lot more work to prevent resource consumption by malicious users than merely limiting who is allowed to set sort_mem/work_mem. If you could implement per-user/per-connection limits on things like processor usage or disk space consumption, I think that would be useful to some users (e.g. people offering PG in a web hosting environment).
-Neil
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