Oracle and MySQL with InnoDB engine are the ones who can really do Index Only, aka Covering Index, in a high OLTP (where readers do not block writers). SQL Server can also do it, but with reads blocking writes (except with snapshot isolation). PostgreSQL can also do it, with non-blocking reads, but not efficiently when the tables have concurrent modifications.
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[ 3.1 ms ] story [ 15.5 ms ] threadOracle and MySQL with InnoDB engine are the ones who can really do Index Only, aka Covering Index, in a high OLTP (where readers do not block writers). SQL Server can also do it, but with reads blocking writes (except with snapshot isolation). PostgreSQL can also do it, with non-blocking reads, but not efficiently when the tables have concurrent modifications.