[dspillett]

It works this way in SQL Server too, and some other DBs, if you have a clustered index (usually recommended). The clustering key is included in all non-clustered indexes on the table. Not that this doesn't mean NCIs inherit any extra fragmentation potential from the clustering key, as it is effectively INCLUDed and not considered by of the key of the supporting index.

Postgres tables are more like what SQL Server calls a heap table (one without a clustering key). Some of the issues that make clustered tables the standard recommendation in SQL Server are very similar to those that make VACUUM a requirement in postgres. IIRC postgres tables are more efficient than SQL Server's heap tables in most cases because they are the only option so are actively optimised for, where in SQL Server head tables are generally (in all but the few circumstances where they are more efficient) considered a second class type.