[CI] Question about sparse primary indexes

[Alexander Rasin]

Although this is a question that came up in different context (of my DBD 
paper), I think this is the right group of people to ask :)

Consider: CIs, in some sense, are like sparse secondary indexes which we 
think makes them cool and unique. The question is this:

Do you know of any row-based-database that supports sparse *primary* 
indexes?  Looking through MS SQL Server, it appears that primary index has 
to be unique, and if it isn't unique, SQLServer will make it unique by 
extending it.
I don't seem to remember postgresql supporting sparse primary index.

So the question is - if sparse indexes are so cool and easy to do 
when we're dealing with a primary index and data is sorted, how come 
they are not used?  Or is it possible to have them in postgres/MS and I 
just don't know how?


Alex