Dense composite supports adding records where only a prefix of all the component specifying the key is defined. In other words, with:
you can insert
You cannot however select that column specifically (i.e, without selecting column (2, '192.168.0.1', 80, 'http') for instance).
This ticket proposes to allow that though 'null', i.e. to allow
It would then also make sense to support:
as an equivalent to the insert query above.