Re: 9.2 and index only scans

Craig Ringer, 28.08.2012 15:04:

>> In Oracle an index (entry) has the information about transactional visibility.
>
> Wow. Doesn't that mean that indexes are insanely expensive to update,
> since each index (and possibly also the table its self) needs
> updating?
>
No, I don't think so.

It's the same mechanism that is used to maintain visibility for table rows.

This information is part of the database block. Blocks for indexes are treated the same way as blocks for tables, so
it'sa uniform implementation. 

It's maintained in a structure called "ITL" (Interested Transaction List) but I think this is getting a bit off-topic
now;) 

If you are interested, this is documented in the Oracle concepts manual:
http://docs.oracle.com/cd/E11882_01/server.112/e25789/consist.htm

Thomas