We have a GeoServer layer backed by a PostGIS table with 22 million smallish features covering the continental US. We're using GeoServer to serve the features as a WMS layer to a client application that is only interested in the data at large scales (close-up). To avoid running queries that return millions of features, I've used a style that shuts off display of all features at smaller scales. This gets the job done, but it's annoying not to be able to get a preview of a larger area (e.g. in GeoServer's OpenLayers layer preview).
Is there some easy way to tell GeoServer, "at small scales, just draw some features"? I'm not really picky about which features so long as the map generally appears to have data wherever we have data. A random subset of them would be fine so long as it was "spatially random" (if that makes sense).