The key question here is whether filtering functions/operators at the level of extensions is a good design. It seems to me like a reasonable compromise between flexibility and ease of use, but others might see it differently.
I like that, but currently we handle things in terms of Schemas. It would be strange to have differing ways of specifying groups of objects. Maybe that's not a problem, but we'd probably need to analyse that to make sure it didn't make things more complex.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services