Well, for some databases, yes, this is "clustering", and there's a command to deal with it ([0] is postgres, which doesn't do this particularly well, other flavours of database have similar commands).
But it's the usual thing with databases: if you're trying to optimise performance by hacking the engine, then your schema probably needs a good hard looking at.
I don't consider being aware of your data distribution to be "hacking the engine". It's part of a good DBA's job. For any data set of appreciable size you can't treat the data as a black box. Even if you're not clustering the underlying table in postgres the performance of your indices will be heavily affected by data layout.
I agree. Hacking the ID field to include index data so the storage clustering is a bit more performant is not "treating the data as a black box" to my mind.
