Is there any point with a combined index on all columns in a table? I have stumbled upon several cases of indices set on all columns of a table. In my mind this will be as efficient as the full table scan that would occur without the index. Is this correct or is there any benefit of such an index? I have seen this in Active Record on top of Postgres so this setup is my primary concern. I also wonder if and how this may differ between the different databases.
The index is a sorted structure - if you need only a sufficiently small portion of the data in the table, it could be fetched from the index more efficiently.

This needs a few prerequisites, though:


PostgreSQL version 9.2 or newer, as index-only scans appeared in this version
the index supports the query (the order of the columns of the index decides this)


Also worth noting that even index-only scans must visit the underlying table (to be precise, its visibility map, and in cases the heap itself) to figure out if the rows to be returned are still visible to the current transaction.  This has to be also considered when determining what 'sufficiently small' means above.